I wouldn't just give up on it due to anything written here - definitely plead your case. In my experience, there are circumstances in which it makes sense for the employer to retain some control over commissions in extraordinary circumstances. A flat cap seems odd though, so I would take a hard look at the language in this contract clause. The real questions in this situation are, how much you did to actually bring in this work, how much you "own" this work (meaning, if you bolt, will it follow you), how much potential for future similar opportunities - essentially, how much leverage do you have and how willing are you to fight for a bigger cut?