I’m a firm believer in relationships maintenance as well as in paying it forward.
But I respect those who insist on contributing and will not fight their will too hard. That mostly because I know “I will get them back”, just so I can feel I did things out of friendship and not because of the reward. The reason I respect their will is because I don’t know if they’ve been burnt before when not giving something in return.
Yet, when I worked alongside homeless people, for a charity (Emmaus, there’s one in Finland too AFAIK) one of the ground rules as instilled by its founder was that those in need needed to give/contribute something. No matter how small that contribution was, it was important they realised that nothing was for free. That was mostly because then they would value what they got more.
I don't give money to beggars unless they are doing something, anything at all to show that they are willing to offer more than a hand out for coins.