I think this is not a proper way for refund, if we enter negative amount for refund then there are lots of chance to get confuse during refund mount so that the calculation becomes incorrect.
So can you suggest me any other method.
If there is no module is build for refund amount then inform me so that we can build a separate module for refund amount, because in Indian market advance payment is most necessary before starting the treatment, once we recieve the advance money then some cases we have to refund the money to the patient after the treatment.