You have some arguments that you can assert defensively.
The first would be that the Broker waived the commission orally by having the money paid out on other issues.
The second would be that there is a written agreement to accept the commission from other sources, and this fell through as a result of impossibility.
Thus, arguably, there is no commission owed, or if it is owed, it is certainly only owed on the original amount with no interest. These defensive arguments are not strong given the fact that you have ostensibly agreed to him in your communications that the money is owed.
Again, I think your best move is to get the money together and tender the original commission as a settlement.
Either way, court litigation
or arbitration, it's not exactly cheap. Arbitration costs around $1000 - $5000, excluding your attorney's fees if you choose to hire an attorney. Court costs are expensive as well, with there being a $250 - $450 charge for each motion filed with the court. So, if you can round up the money to pay the Broker off, that will likely be the best route for you.