Getting Error while entering the discount amount while applying the receipt - Discount Taken is grea

Getting Error while entering the discount amount while applying the receipt - Discount Taken is greater than the discount available- APP-AR-11248."

1. There is an Open AR Invoice Transaction -- eg 100 dollars

2. Trying to create a Receipt for 80 dollars as receipt apply and 20 I want to give the discount to my customers

3. Entered 80 dollars in amount field after apply button in Receipt

4. Entered 20 dollars in Discount field, getting the error "Getting Error "Discount Taken is greater than the discount available- APP-AR-11248."

How can it be allowed to enter the Discount amount or any other setups.

Another question : Discount can be calculated at the time of Creating Transactions or only the way we can enter the Receipt Amount and Discount amount manually at the time of Entering Receipts against the Open Transaction.