When going to work today, I stopped to buy a bottle of water in a supermarket held by an anglophone from Nigeria. When it was time to ask him how much is the price of the bottle of water, as a good francophone I asked him : "How much do I owe you ?". And he started laughing. And I'm still wondering why. Isn't it correct ? I know I could say something like "how much is the price of..." or "how much do I give you..." but isn't it also correct to use the verb to owe ?