Appellate Practice Attorney serving New York, NY
Assuming that the contract was otherwise valid (e.g. he was competent, he received consideration for his promise, there was no duress, you did not defaurd him, etc.), yes. There is no need for notarized signatures to make this contract valid. In fact, most contracts do not even require a writing to be valid (although a writing obviously helps provide proof that a contract was made.)
One caveat. Most installment payment contracts contain what is known as an "acceleration clause", which provides that if one installment is missed, the entire amount becomes immediately due and owing. If you didn't have one in your contract, you can only sue for the amount which is due as of now. If the whole amount is not yet due, you can either wait until it is and then sue for it, or you can sue for what is owing now and either amend your complaint later (if the court will allow it) or sue again when the rest comes due.
Answered on Dec 09th, 2013 at 3:08 PM