You issued an Invoice for $2,030.00 but your customer only paid you $2,000.00 and you don’t want to recover the difference.
First knock off the Invoice with $2,000 as usual. Your Debtor Reports and Statements will still show an outstanding debt of $30.00. Call for another new receipt. Enter the Total Receipts as $30.00 but this time, instead of selecting your usual bank account, you can charge it to “Bad Debt Written Off” or you could even define your own Account as “Discounts” if you prefer. Note that there are many methods of doing this, you could also issue a Credit Note.