Just FYI, I was dealing with a situation where a bank and the SBA approved an incorrect forgiveness amount due to how the forms had to be completed (more precisely, it was confusing, even to me, how they wished the EIDL advance to be reported). I reached out to the bank to see what needed to be done to correct it, and the bank said there was nothing that could be done since the SBA had already approved and remitted the forgiven amount to the bank.
I did not accept that. Since it is nearly impossible to reach anyone at SBA, I ended up writing a letter to the SBA, with full support and an explanation of WHY the forgiveness amount was incorrect. While it took nearly two months, I just saw today that the SBA forgave the additional amount, remitted the extra amount to the bank, and the remaining PPP liability has been reduced at the bank.
If you are working with clients and the forgiveness amount is incorrect, do not accept it as gospel when a bank says nothing more can be done once the SBA issues its approval. It is not final, and it can be adjusted after-the-fact.