A buddy of mine who works as an acquisition manager for a larger investor says he gets a 3% commission on either side of a wholesale deal -- once for when he contracts the purchase, and another when he closes with the final buyer. But this guy's commission comes out of the pocket of the investor that he works for, not the seller or the buyer.
So I think you have a lot of leeway. Your PM gave you a lead that you ultimately want to move on, which is great, but he didn't tell you what his claim in the process was. My opinion is that you have to advocate for yourself, meaning that your PM did not establish that he was doing it for money and you had an existing relationship so he left it vague. That leaves the choice entirely up to you and, I think, puts you in a "tipping for service" scenario. I neither like nor understand the American practice of tipping people for doing their jobs or just for doing things they wanted to do, but certainly it is widespread.
What would you tip for this service? Maybe you could offer him something like1.5X( 2-3 hrs) worth of his estimated hourly wage as your PM? As "overtime" pay? I guess in the end you just have to find something you're comfortable with. And don't be afraid to ask your PM if what you're offering would be satisfying to him given his effort.