If you have already created and posted an A/R Invoice, you can write it off with the following steps:
1) Run the Accounts Receivable Adjustments/Write-off Entry screen (AR0074)
2) Click the F5 key (or the New Record icon, below) to create a new entry
3) Choose the appropriate Type for the adjustment (NOTE: if you need a new type, you can create that on the Code Table Maintenance screen / KC0004 - the code table is AR.TRANS.TYPE).
4) For the Post Date, you can use any date you like - you may want to use the original invoice date, as long as that period is still open.
5) If the refund amount is $120, enter 120 as the amount. Positive amounts reduce the A/R invoice balance.
6) Enter the G/L account to be credited (it must be the same G/L account on the invoice).
7) Enter the A/R Invoice number and the write-off amount (this will match the Total amount if you are only writing off one invoice).
8) When you click OK, the transaction will post; there is no register/update process. See screenshot below: