Your argument would be that nothing materially changed in your job performance, and that the time period was so short between the signing of the amendment and your termination that the company didn't deal with you in good faith. Basically they used the contract amendment to defraud you out of a bonus/commissions you would've otherwise been entitled to keep but for their deceit.

There's a lot of "it depends" associated with this argument based on the facts, but on the surface you at least have an argument.