So the charges are the difference between the Purchase and the Refund - these types of charges are known as Handling Fees or Restocking Fees as they are charged by the Supplier and not by a bank.
It is being assumed that the Purchase Invoice (500) has been paid and that is why you are receiving a refund.
First you need to take up a Debit Note so that the Inventory levels can be adjusted. The Debit Note details would match the Purchase Invoice
Then you would do the Receive Money (480) with 500 going to Supplier account and -20 going to an expense. (Note the minus sign)
The Suppliers account now looks like this
For a fuller description read this Guide
https://forum.manager.io/t/debit-notes-for-supplier-returns-and-refunds/7426