You should have told the bank NO. That was your fault.
It is not legal to require 3rd party approval for the commissions.
Remember that compensation is BROKER TO BROKER and you can NOT hold a gun to their head to require them to cut the published commission rate.
Watch VAR's Lem the Legal Eagle talk about short sales and commissions.
Next time, tell the bank no, and watch them fold their deck of cards. Nowhere does it say that we have to work for free. We have our rates and it the bank doesn't want it, they can reject the offer.