You have not stated your part in this transaction. Are you the Buyer, Listing Agent, Seller?
The commission rate should have been negotiated in the Listing Agreement and the split on the MLS. If it was a Short Sale the amount paid would also be agreed to by the bank.
If the Buyer's agent got 2,5%, then someone lost out. If it was the Listing Agent, the broker needs to rectify with the other broker. If the bank allowed 2.5%. they must have approved the HUD1, so I would let it slide.