I would say that you do not have a social contract with somebody who approaches you soley for money. Therefore, you are not required to respond to him, nor acknowledge him, anymore than you would be required to respond to somebody aggressively selling something you hadn't asked for.
If he was otherwise social and pleasant, then you should acknowledge him. There is a vendor who sells the homeless newspaper The Contributor near our post office. It has a new edition once per month, so I buy it once per month. But whenever he sees me coming to get the mail he says hello, and wishes me a pleasant day when I'm leaving, even though I don't buy the paper. That's a social contract that I acknowledge with similar responses.