Refund Only What is Charged in Bills & Charges

It should not be possible to refund more to a guest credit card or a cash payment more than has been taken in. 

It should also not be possible to make a negative adjustment to a charge code that is more than what has been posted on that bill as a positive amount.

For example, a desk agent can mistakenly adjust a charge code & put -$300 in the new amount rather than $0. This will cause a discrepancy in that more was adjusted off the folio than was initially charged. Then it is possible to refund the guest credit card $300 that was never taken as payment.