I had 2 instances of this today. Tony, a senior man at a big bank asked if I'd do a free hour for him and his mteam on a particular aspect of networking. No problem. When I arrived they asked how they could help me..this was even before I had given any advice.
I hope I helped and then mentioned the introductions I was loooking for. I know I will get more business from this 1 hour.
This evening I did a presentation at a discounted price. The organiser knew this and her gratitude showed when she asked if I could do some more work at a commercial rate.