reducing the emoluments after the service is provided

I am providing an educational service to a school for which I am paid half yearly basis on a fee per student basis. Initially the fees was agreed to be X Rs .Now when my payment is due the principal asked a certain percent commission which is very huge sum from the fees to which I disagreed so now she is going back on her promise and is saying that the management thinks this is a huge fee and asked me to reduce the fee to Y Rs which would be potential loss for me. I have not signed a contract with the school. Since I don't have any agreement or paperwork can I take any legal action against the principal as I am sure of her involvement???