How to deal with stingy friend.?

Me and my friend are on the opposite whenever it comes to money. My friend, I would say high-cl wealthy, is always so stingy, and me, on completely on the other side, not flat broke, but hanging in there. Well, example, everytime we go lets say to a store, I might grab a few things, and he will too, and whenever it comes time to check out our stuff, he says, lets just put it all together, and I will pay you back, he usually never pays me back, or i don't know if he is acting stupid, because he says he doesnt remember. Normally it wouldnt bother me, but it has been happening too often. Other times, he might just tell me instead, hey, get me this. Feeling burdened, I usually grab him w/e he wants, thinking he will pay me back. And when I asked him to pay me back, he was like don't worry it was only 20-30 bucks. That doesnt seem like much, but I'm not wealthy so that can get me things I actually might need. Help!!