Comment 3 for bug 1810429

I have determined this is a regression introduced by 4efaa63 (see LP 1749994)

Because of commit 4efaa63 the "Apply Payment" button will be disabled if the payment amount is 0. The refund functionality relies on clicking the "Apply Payment" button when the amount is zero, so it's impossible to issue a refund.

Reverting commit 4efaa63 resolves this issue but obviously doesn't solve both bugs.