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The commission agreement was made when he was employee. Unless there is some specific agreement that the arragment would continue in the future, the commission arrangement ended when he left your employ. Also, someone seeing your work does not make the person who brought the work initially become the procuring agent because a stranger decided to use you because of the work they say. You owe this person nothing. If the person wants to sue let him as I do not think his case would survive a motion to dismiss on the facts you laid out.
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