Thank you, but this reply missed the point. The issue is that when marking a Vendor Bill as Paid, QuickBooks doesn't allow the user to apply a Credit Memo if it is identical to the amount due.

QBO now requires a non-zero dollar value to be entered in the Amount field before the "Mark as Paid" feature works.

The workaround, as explained by other QB employees here, seems to be to *hire an accountant* to create a Journal entry.

This task of applying a Credit Memo that exactly offsets the Bill's balance seems simple, yet the suggested solution is so complicated, I suspect I must be missing something obvious.