I want extra validation on a form, how can this be achieved

Within AR Adjustments, users can write-off any invoice balance within their approval limits.

I want to prevent users from being able to close the transaction.

For example if my upper approval limit is 10 gbp, I want to be able to write-off/adjust an invoice for this amount UNLESS this is the total amount remaining on the trx because this will then close the transaction.

Is this possible?