Fix:

Discount button was greyed out due to wrong check.
  Show was sending customer as empty dictionary which triggered an error in input as id was a required column

barker/routers/voucher/show.py

@@ -84,7 +84,7 @@ def voucher_info(item: Voucher):
         "lastEditDateTip": item.last_edit_date.strftime("%d-%b-%Y %H:%M:%S"),
         "billId": item.full_bill_id,
         "table": {"id": item.food_table_id, "name": item.food_table.name},
-        "customer": {"id": item.customer_id, "name": item.customer.name} if item.customer is not None else {},
+        "customer": {"id": item.customer_id, "name": item.customer.name} if item.customer is not None else None,
         "settlements": [],
         "narration": item.narration,
         "reason": item.reason,

bookie/src/app/sales/home/sales-home.component.ts

@@ -69,7 +69,7 @@ export class SalesHomeComponent implements OnInit {
   }
 
   discountAllowed(): boolean {
-    return this.auth.user.perms.indexOf('discount') === -1;
+    return this.auth.user.perms.indexOf('discount') !== -1;
   }
 
   showDiscount(): Observable<boolean | { id: string, name: string, discount: number }[]> {