I used to do something like that with the old man selling newspapers at the Ramada signal (as you're coming from Al Sadd). I got into the habit of buying something and giving him an extra 5 or 10 Riyals, at other times just giving him money without buying anything.
He got used to it and when he'd see my car at the lights would come over and decide he wasn't going to even try to give me back any change for a purchase or he'd just stand there smiling waiting for me to give him his dues.
So I cut the habit and what I occasionally do now is hand these guys a bottle of water if I have it or let them keep 2 or 3 Riyals out of the 5.