What payment to be made for transaction related to before GST era

If a company "A" has issued order for day to day maintenance work on firm "B" on dated 01/11/2013 for rs. 10+2.36(service tax @ 12.36%) for one year.
Firm B while submitting invoice didn't produced other required documents so company A could not process firm B's payment.
now on 2022 firm submitted all documents and asking for its dues payment.
now on the same service GST is 18%.
so what the company should do in this situation. what amount it should pay to firm and please quote relevant provision in this regard????
thanks in anticipation