Here are the minimal setup required for reproducing this bug.
Steps to reproduce:
- go to users app and click on some active user
- open fees/fines section and click on a "Create fee/fine" button
- fill the form based on setup from the pre-conditions
- click a "Charge only" button
- select just created item and open a details page
- click on "Pay" button and enter Payment amount less than Selected amount
- chose "Payment method" and click "Pay"
- REPEAT two previous steps without reloading the page.
- reload the page.
"Pay" button will be disabled when payed amount will be more than billed amount.