Giving does something immediate and slightly strange to your own accounting. The instant you hand something over, you have proved to yourself that you had enough to hand over — and that lands harder than any balance ever does.
People with very little give anyway, all the time, and describe feeling less poor afterwards rather than more. Not because it came back. Because the act contradicted the story running underneath.
None of which obliges anybody to give what they cannot spare. But the return on the small ones is faster than expected, and it does not arrive by post.