"...me lo pasas después..."

I guess the concept of credit at stores is not dead in Guatemala.  As many who travel throughout Latin America know, and no doubt in other parts of the world, shops don't carry a lot of extra cash.  I was bike-riding the other day, and halfway through I stopped at a corner store in a plaza we frequent.  I've been in the store maybe five times, and not really ever made conversation there.  All I had on me was a 100 quetzal bill (about 12 USD) and was buying a bottle of water that cost Q3.50 (about 40cents US).  The cashier asked me if I had anything smaller, and I didn't, and she just said "me lo pasas después"..."you can give it to me later".  I looked at her in disbelief, thanked her, and walked out with my water without her taking my name or anything.  I fully intend on paying her the next time I go to the store.  Anybody else encountered this?