to discount a previous balance, you have to do a write off. To adjust a bill you are creating you can do a credit or you can use a fee arrangement to adjust total charges. The fee arrangement would be done in client information under the arrangement tab. 

Thanks!

A write-off is only one of several options and could possibly not be the best solution depending on the circumstances.

A Write-off is intended for a bad debt scenario where you are needing to close a specific invoice balance for a specific reason.  You can define that reason as you wish. 

You could also use a Credit against the total amount due (the previous balance) for the client. Other methods would be Decrease Invoice Details or Discount for specific invoices.  All of these are transaction choices in Account Receivable.