I have no source for what I’m about to say, but I think it makes sense. Sometimes, when we do a mitzvah or when we are on are way to do a mitzvah, G-d throws us a curveball, and makes doing the miztvah much more difficult. Perhaps, it’s because He wants to give us more reward for it. That once He sees we are doing a mitzvah, He throws in a few more opportunities (that we sometimes see as obstacles) to get even more mitzvos. Or, it’s because it is a mitzvah that we would do anyway, even if G-d had not commanded it, because it’s a nice thing to do (like giving tzedaka), so G-d wants to see if we are doing it just because it’s nice to do, or because we love G-d, and G-d commanded us to do it, and no matter what obstacle we may face in our way of doing this mitzvah, we will do it anyway to show our loyalty to Him.
Regardless, though, I am sorry you went through all this trouble when trying to do a mitzvah, and I can see how much of a let down it can be. You are doing an amazing thing by giving tzedaka even without having that much money. Stay strong, and may you always have the ability to give tzedaka with a smile!