As long as it was signed by him, it would be binding on him, with or without your signature. He could raise a defense that the written agreement is not binding since you never agreed to it. That would come down to whoever the judge believes. A verbal agreement is just as enforceable as a written agreement, so if the written terms were the same as your verbal terms, then it might not make a difference.
If your friend is the person obligated by the K to make the payment, the K is good as against your friend without your signature. Still, there could be other reasons that the K is not enforceable and at best you could only enforce the terms stated.
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