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If you have a third party witness with knowledge that the contracting party's revenue was greater than reported to you, then you can sue for the difference. The fact that you were paid a percentage of what you thought the value of the loan was, proves that there was a commission involved, and assuming you have some sort of statement from the other party in writing of what you were being paid for on the checks, then that would prove the contract.
It really depends on the quality of your evidence. I can see the possibility of proving the breach, but without reviewing all of your documents and accounts, there's no way to definitively state that you have a good case.
For a civil litigation attorney referral in your state, see: http://www.abanet.org/legalservices/lris/directory/ and www.martindale.com.
Hope this helps.
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