1. Enter a payment from customer through AR-C
2. Leave it as an open DEPOSIT
3. Save it
4. Enter same customer code
5. enter Negative of previous payment
6. Select Reversal
See:
This PDF.
The Reversal posted to 21500 should be a debit (since it is a credit, 21500 now shows $2000 on deposit)
The other side should also be changed accordingly.