Sounds like a matter of another VM with a questionable business practice.

They can not unilaterally change the contracted price. Although you may have a more difficult time winning your case after having deposited the check, you still have shot at winning your case. Hopefully the check did not have the words "Paid in full" or similar verbiage written on it or on a memo to which it was attached. If so, it becomes very difficult to win (but not impossible). I have won cases on that basis. The court ruled for my client (defendant) on the basis that the plaintiff had presented the check for payment which check was clearly marked "Payment in full" by my client..

If there was no mention of a dispute over the contract price prior to receiving the check or in the materials sent with the check or if there was no indication that the check was payment in full, you can argue that you accepted the check as partial payment only. The court could rule either way, but at least you will have a chance to vent your spleen. 
If you elect to follow this route, you should send your client a written explanation sent certified mail return receipt requested that you are accepting the check only as partial payment, and in the letter you should make demand for the outstanding balance. 

 For future reference return the check uncashed if you feel that the defendants will still be solvent by the time a small claims court renders judgment against them. In the instant case it sounds like they sent you a check for pennies on the dollar, and returning the check would not have been ay great loss. If they are insolvent, you may have to accept the payment, and satisfy yourself that you got half a loaf.