Hi Ian,
If you record the Deposit using the Supplier name in the Received From field and your Accounts Payable/Trade Creditors account in the From Account field this will create a contra debit entry which you will see in the Pay Bills screen like a Bill. Allocate the original credit to this Deposit entry.
If returning the balance of the Supplier payment you can use the Write Cheques function allocating the payment again to your Accounts Payable/Trade Creditors account. This will create a contra credit entry and again in the Pay Bills screen you can allocate it to the remaining debit entry created by the Deposit transaction.
Hope this all makes sense.
Suzanne
Very welcome