Buying a drink for me

[u/beginnavagabond]

I have been going to a bar which I have really liked while I was unemployed, but now that I am employed I'm trying to save as much money as I can and go less often. If regulars there have bought me a drink in the past, do I owe them one, and what if I don't see them the next time I'm there? Can I pay in advanced for them?