diff --git a/Backend/ERPCore/Domain/Entities/SalesInvoiceLine.cs b/Backend/ERPCore/Domain/Entities/SalesInvoiceLine.cs
index 05310de..e7e4beb 100644
--- a/Backend/ERPCore/Domain/Entities/SalesInvoiceLine.cs
+++ b/Backend/ERPCore/Domain/Entities/SalesInvoiceLine.cs
@@ -1,3 +1,5 @@
+using ERPCore.Domain.Enums;
+
 namespace ERPCore.Domain.Entities;
 
 public class SalesInvoiceLine
@@ -22,6 +24,7 @@ public class SalesInvoiceLine
     public decimal UnitPrice { get; set; }
     public decimal BaseCost { get; set; }
     public string PriceSource { get; set; } = string.Empty;
+    public SalesDiscountMode DiscountMode { get; set; } = SalesDiscountMode.Percentage;
     public decimal DiscountPct { get; set; }
     public decimal DiscountAmount { get; set; }
     public decimal NetUnitPrice { get; set; }
diff --git a/Backend/ERPCore/Domain/Entities/SalesSlipLine.cs b/Backend/ERPCore/Domain/Entities/SalesSlipLine.cs
index c0079e3..0eb3de2 100644
--- a/Backend/ERPCore/Domain/Entities/SalesSlipLine.cs
+++ b/Backend/ERPCore/Domain/Entities/SalesSlipLine.cs
@@ -1,3 +1,5 @@
+using ERPCore.Domain.Enums;
+
 namespace ERPCore.Domain.Entities;
 
 public class SalesSlipLine
@@ -22,6 +24,7 @@ public class SalesSlipLine
     public decimal UnitPrice { get; set; }
     public decimal BaseCost { get; set; }
     public string PriceSource { get; set; } = string.Empty;
+    public SalesDiscountMode DiscountMode { get; set; } = SalesDiscountMode.Percentage;
     public decimal DiscountPct { get; set; }
     public decimal DiscountAmount { get; set; }
     public decimal NetUnitPrice { get; set; }
diff --git a/Backend/ERPCore/Domain/Enums/SalesDiscountMode.cs b/Backend/ERPCore/Domain/Enums/SalesDiscountMode.cs
new file mode 100644
index 0000000..c6d8f79
--- /dev/null
+++ b/Backend/ERPCore/Domain/Enums/SalesDiscountMode.cs
@@ -0,0 +1,7 @@
+namespace ERPCore.Domain.Enums;
+
+public enum SalesDiscountMode
+{
+    Percentage = 1,
+    FixedAmount = 2
+}
diff --git a/Backend/ERPCore/Dtos/Sales/SalesInvoiceDtos.cs b/Backend/ERPCore/Dtos/Sales/SalesInvoiceDtos.cs
index b90fd30..f8f8f5e 100644
--- a/Backend/ERPCore/Dtos/Sales/SalesInvoiceDtos.cs
+++ b/Backend/ERPCore/Dtos/Sales/SalesInvoiceDtos.cs
@@ -6,8 +6,8 @@ namespace ERPCore.Dtos.Sales;
 public sealed record SalesInvoiceLineDto(
     int SalesInvoiceLineId, int ItemId, string Description, decimal Qty, decimal FreeQty, int UomId,
     int WarehouseId, decimal UnitPrice, decimal BaseCost, string PriceSource, decimal DiscountPct,
-    decimal DiscountAmount, decimal NetUnitPrice, decimal LineTotal, decimal TaxPct, decimal TaxAmount,
-    bool IsFreeIssue, int? ParentLineId);
+    decimal DiscountAmount, SalesDiscountMode DiscountMode, decimal NetUnitPrice, decimal LineTotal,
+    decimal TaxPct, decimal TaxAmount, bool IsFreeIssue, int? ParentLineId);
 
 public sealed record SalesInvoiceTotalsDto(
     decimal Subtotal, decimal DiscountTotal, decimal TaxTotal, decimal GrandTotal,
@@ -33,8 +33,10 @@ public sealed class CreateSalesInvoiceLineRequest
     [Range(0, double.MaxValue)] public decimal FreeQty { get; set; }
     [Range(0, double.MaxValue)] public decimal? UnitPrice { get; set; }
     public bool AllowManualPriceOverride { get; set; }
+    [Required, EnumDataType(typeof(SalesDiscountMode))] public SalesDiscountMode DiscountMode { get; set; } = SalesDiscountMode.Percentage;
     [Range(0, 100)] public decimal DiscountPct { get; set; }
     [Range(0, double.MaxValue)] public decimal DiscountAmount { get; set; }
+    [Range(0, double.MaxValue)] public decimal DiscountValue { get; set; }
     [Range(0, 100)] public decimal TaxPct { get; set; }
     public bool IsFreeIssue { get; set; }
     public int? ParentLineId { get; set; }
diff --git a/Backend/ERPCore/Dtos/Sales/SalesSlipDtos.cs b/Backend/ERPCore/Dtos/Sales/SalesSlipDtos.cs
index eab27c9..11a4597 100644
--- a/Backend/ERPCore/Dtos/Sales/SalesSlipDtos.cs
+++ b/Backend/ERPCore/Dtos/Sales/SalesSlipDtos.cs
@@ -6,8 +6,8 @@ namespace ERPCore.Dtos.Sales;
 public sealed record SalesSlipLineDto(
     int SalesSlipLineId, int ItemId, string Description, decimal Qty, decimal FreeQty, int UomId,
     int WarehouseId, decimal UnitPrice, decimal BaseCost, string PriceSource, decimal DiscountPct,
-    decimal DiscountAmount, decimal NetUnitPrice, decimal LineTotal, decimal TaxPct, decimal TaxAmount,
-    bool IsFreeIssue, int? ParentLineId);
+    decimal DiscountAmount, SalesDiscountMode DiscountMode, decimal NetUnitPrice, decimal LineTotal,
+    decimal TaxPct, decimal TaxAmount, bool IsFreeIssue, int? ParentLineId);
 
 public sealed record SalesSlipTotalsDto(
     decimal Subtotal, decimal DiscountTotal, decimal TaxTotal, decimal GrandTotal,
@@ -32,8 +32,10 @@ public sealed class CreateSalesSlipLineRequest
     [Range(0, double.MaxValue)] public decimal FreeQty { get; set; }
     [Range(0, double.MaxValue)] public decimal? UnitPrice { get; set; }
     public bool AllowManualPriceOverride { get; set; }
+    [Required, EnumDataType(typeof(SalesDiscountMode))] public SalesDiscountMode DiscountMode { get; set; } = SalesDiscountMode.Percentage;
     [Range(0, 100)] public decimal DiscountPct { get; set; }
     [Range(0, double.MaxValue)] public decimal DiscountAmount { get; set; }
+    [Range(0, double.MaxValue)] public decimal DiscountValue { get; set; }
     [Range(0, 100)] public decimal TaxPct { get; set; }
     public bool IsFreeIssue { get; set; }
     public int? ParentLineId { get; set; }
diff --git a/Backend/ERPCore/Services/SalesInvoiceService.cs b/Backend/ERPCore/Services/SalesInvoiceService.cs
index d49252e..9736cbe 100644
--- a/Backend/ERPCore/Services/SalesInvoiceService.cs
+++ b/Backend/ERPCore/Services/SalesInvoiceService.cs
@@ -182,8 +182,7 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
             var priceSource = resolved.PriceSource;
 
             var gross = r.Qty * unitPrice;
-            var discountAfterPct = gross * (r.DiscountPct / 100m);
-            var discountTotal = discountAfterPct + r.DiscountAmount;
+            var discountTotal = CalculateDiscount(gross, r.DiscountMode, r.DiscountPct, r.DiscountValue, r.DiscountAmount);
             var netUnit = r.Qty > 0 ? (gross - discountTotal) / r.Qty : 0m;
             var lineTotal = gross - discountTotal;
             var taxAmount = lineTotal * (r.TaxPct / 100m);
@@ -201,6 +200,7 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
                 PriceSource = priceSource,
                 DiscountPct = r.DiscountPct,
                 DiscountAmount = discountTotal,
+                DiscountMode = r.DiscountMode,
                 NetUnitPrice = netUnit,
                 LineTotal = lineTotal,
                 TaxPct = r.TaxPct,
@@ -224,6 +224,18 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
         invoice.BalanceAmount = invoice.NetPayable;
     }
 
+    private static decimal CalculateDiscount(decimal gross, SalesDiscountMode mode, decimal discountPct, decimal discountValue, decimal legacyDiscountAmount)
+    {
+        var computed = mode == SalesDiscountMode.FixedAmount
+            ? discountValue
+            : gross * (discountPct / 100m);
+
+        if (computed <= 0m && legacyDiscountAmount > 0m)
+            computed = legacyDiscountAmount;
+
+        return Math.Min(gross, Math.Max(0m, computed));
+    }
+
     private static SalesInvoiceSummaryDto MapSummary(SalesInvoice x) => new(
         x.SalesInvoiceId, x.InvoiceNo, x.InvoiceDate, x.CustomerId, x.CustomerSnapshotName,
         x.WarehouseId, x.InvoiceType, x.Status, new SalesInvoiceTotalsDto(
@@ -233,5 +245,5 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
         x.SalesInvoiceId, x.InvoiceNo, x.InvoiceDate, x.CustomerId, x.CustomerSnapshotName, x.CustomerSnapshotTaxNo,
         x.WarehouseId, x.InvoiceType, x.Status, x.CreatedBy, x.CreatedAt, x.UpdatedAt,
         new SalesInvoiceTotalsDto(x.Subtotal, x.DiscountTotal, x.TaxTotal, x.GrandTotal, x.RoundOff, x.NetPayable, x.PaidAmount, x.BalanceAmount),
-        x.Lines.Select(l => new SalesInvoiceLineDto(l.SalesInvoiceLineId, l.ItemId, l.Description, l.Qty, l.FreeQty, l.UomId, l.WarehouseId, l.UnitPrice, l.BaseCost, l.PriceSource, l.DiscountPct, l.DiscountAmount, l.NetUnitPrice, l.LineTotal, l.TaxPct, l.TaxAmount, l.IsFreeIssue, l.ParentLineId)).ToList());
+        x.Lines.Select(l => new SalesInvoiceLineDto(l.SalesInvoiceLineId, l.ItemId, l.Description, l.Qty, l.FreeQty, l.UomId, l.WarehouseId, l.UnitPrice, l.BaseCost, l.PriceSource, l.DiscountPct, l.DiscountAmount, l.DiscountMode, l.NetUnitPrice, l.LineTotal, l.TaxPct, l.TaxAmount, l.IsFreeIssue, l.ParentLineId)).ToList());
 }
diff --git a/Backend/ERPCore/Services/SalesSlipService.cs b/Backend/ERPCore/Services/SalesSlipService.cs
index 96c6a6b..a92a355 100644
--- a/Backend/ERPCore/Services/SalesSlipService.cs
+++ b/Backend/ERPCore/Services/SalesSlipService.cs
@@ -183,8 +183,7 @@ public sealed class SalesSlipService : ISalesSlipService
             var priceSource = resolved.PriceSource;
 
             var gross = r.Qty * unitPrice;
-            var discountAfterPct = gross * (r.DiscountPct / 100m);
-            var discountTotal = discountAfterPct + r.DiscountAmount;
+            var discountTotal = CalculateDiscount(gross, r.DiscountMode, r.DiscountPct, r.DiscountValue, r.DiscountAmount);
             var netUnit = r.Qty > 0 ? (gross - discountTotal) / r.Qty : 0m;
             var lineTotal = gross - discountTotal;
             var taxAmount = lineTotal * (r.TaxPct / 100m);
@@ -200,6 +199,7 @@ public sealed class SalesSlipService : ISalesSlipService
                 UnitPrice = unitPrice,
                 BaseCost = unitPrice,
                 PriceSource = priceSource,
+                DiscountMode = r.DiscountMode,
                 DiscountPct = r.DiscountPct,
                 DiscountAmount = discountTotal,
                 NetUnitPrice = netUnit,
@@ -223,6 +223,18 @@ public sealed class SalesSlipService : ISalesSlipService
         slip.BalanceAmount = slip.GrandTotal - slip.PaidAmount;
     }
 
+    private static decimal CalculateDiscount(decimal gross, SalesDiscountMode mode, decimal discountPct, decimal discountValue, decimal legacyDiscountAmount)
+    {
+        var computed = mode == SalesDiscountMode.FixedAmount
+            ? discountValue
+            : gross * (discountPct / 100m);
+
+        if (computed <= 0m && legacyDiscountAmount > 0m)
+            computed = legacyDiscountAmount;
+
+        return Math.Min(gross, Math.Max(0m, computed));
+    }
+
     private static SalesSlipSummaryDto MapSummary(SalesSlip x) => new(
         x.SalesSlipId, x.SlipNo, x.SlipDate, x.CustomerId, x.CustomerSnapshotName, x.WarehouseId, x.Status,
         new SalesSlipTotalsDto(x.Subtotal, x.DiscountTotal, x.TaxTotal, x.GrandTotal, x.PaidAmount, x.BalanceAmount), x.CreatedAt);
@@ -230,5 +242,5 @@ public sealed class SalesSlipService : ISalesSlipService
     private static SalesSlipDto Map(SalesSlip x) => new(
         x.SalesSlipId, x.SlipNo, x.SlipDate, x.CustomerId, x.CustomerSnapshotName, x.WarehouseId, x.CashierUserId, x.Status, x.CreatedAt, x.UpdatedAt,
         new SalesSlipTotalsDto(x.Subtotal, x.DiscountTotal, x.TaxTotal, x.GrandTotal, x.PaidAmount, x.BalanceAmount),
-        x.Lines.Select(l => new SalesSlipLineDto(l.SalesSlipLineId, l.ItemId, l.Description, l.Qty, l.FreeQty, l.UomId, l.WarehouseId, l.UnitPrice, l.BaseCost, l.PriceSource, l.DiscountPct, l.DiscountAmount, l.NetUnitPrice, l.LineTotal, l.TaxPct, l.TaxAmount, l.IsFreeIssue, l.ParentLineId)).ToList());
+        x.Lines.Select(l => new SalesSlipLineDto(l.SalesSlipLineId, l.ItemId, l.Description, l.Qty, l.FreeQty, l.UomId, l.WarehouseId, l.UnitPrice, l.BaseCost, l.PriceSource, l.DiscountPct, l.DiscountAmount, l.DiscountMode, l.NetUnitPrice, l.LineTotal, l.TaxPct, l.TaxAmount, l.IsFreeIssue, l.ParentLineId)).ToList());
 }