Does Missouri honor in court when you send a check with writing on the back saying if you cash this check you have entered into a new contract? How would I find this out? Please help Thanks

Not really, first is the check hand signed or is it merely computer signed, or is the check just scanned and not even cashed as like taken to the bank.

Even the old "paid in full " trick is seldom accepted in courts any longer

And it will also deal even into what type of contract, why you are paying them money and more.

Give full details for a proper answer on the legal way to proceed