The burden of proof is on the person claiming the existence of the agreement to prove that it actually does exist and is contractually binding. If it's a verbal agreement, yes, there are ways to do that but it's a pain in the petoot. Because of this, I suspect he's blowing a lot of bad-smelling hot air at you, but you should probably retain an attorney just in case.
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Out of curiosity, did anything ever come of this fellow's threat? I suspect by now he has called a lawyer or two and been told that it is not worth pursuing.