Hi - my name is Kirk and I'm a Business
litigation attorney. Thanks for your question.
If you have a contract for a specific product to be provided, and if you provided that, then the contract was likely fulfilled. Also, if there were no change orders to modify the agreement, then the changes requested should have required additional payment in order for you to do the extra work that is outside of the original contract. IF there was no change order, it could be a little more difficult seeking payment for this additional work.
Outside of you and the client reaching an agreement to pay you the amount owed, you'll have to sue, and if you're incorporated, you can only sue with an attorney because individuals can't sue on behalf of another person or corporate entity.
You could sue for anything you're due under the original contract, and you can seek payment for the additional work by claiming that it was a request for new work, and you charged them separately.
Other than suing this client, there's likely no way that you will ever recover your money. ALSO, in order for the client to sue you, he will have to prove that you breached the contract - - which will require an objective review of the contract and then what was produced by you. It would be up to a judge to determine whether or not your work complied with the terms of the contract. If so, you'd win, if not, they'd win.
However, the client may be bluffing, and likely should be careful to sue you since you could counter-sue for all of the work that has been done, which is outside of the contract.