diff --git a/Distribution/Models/Invoice.cs b/Distribution/Models/Invoice.cs
index c2a71f2..5985b62 100644
--- a/Distribution/Models/Invoice.cs
+++ b/Distribution/Models/Invoice.cs
@@ -19,6 +19,10 @@ namespace Distribution.Models
         public DateTime? FinalizedAt { get; set; }
         public Guid? FinalizedBy { get; set; }
 
+        /// <summary>Scheme discount redeemed against the PO, subtracted from the billable lines to give TotalAmount.</summary>
+        public decimal DiscountAmount { get; set; }
+
+        /// <summary>Payable amount: billable lines minus DiscountAmount.</summary>
         public decimal TotalAmount { get; set; }
 
         public ICollection<InvoiceItem> Items { get; set; } = new List<InvoiceItem>();