Generally you "write off" a client balance in the Timeslips Accounts Receivable Transactions when there is no hope of receiving payment from the client. Every once in a while, the "hopeless" client finally submits a payment. Now what do you? The balance has been written off in Timeslips so there is nothing to apply the payment to in the Accounts Receivable transactions.
The best way to handle this is to use the Timeslips "Invoice" option in the A/R transaction screen. You can enter the amount of the "invoice" that is equal to the write off amount. You may even wish to enter the original Timeslips invoice number to give you a better audit trail. Since this Timeslips A/R transaction "invoice" will never go to the client, you can enter an explanation for the "invoice" in the Timeslips A/R transaction Description field.