1. Close the adjustment (you will not be able to save it.)
2. Close the invoice.
3. In Accounts Payable, go to Configuration, Business Rules.
4. Highlight General in the left-hand list.
5. On the right-hand side, Change the drop-down for "Allow non-bank transactions to use Cash Accounts" from Never to Ask Each Time.

You will now be able to adjust the invoice. When you attempt to save the adjustment, the system will simply warn you that a cash account is being used, but you will be able to click OK to this warning. Save the adjustment again and post as normal.