When you go a service, (car repair maybe) when that service gives you a bill, at that moment, they are the creditor, and you are the debtor.

If you pay for something in advance, like bus fare, or merchandise that is still at the the store, then there is not debtor/creditor relationship.

Clear as mud to me.

If I go somewhere and order a meal and eat it, and then they hand me the bill, I owe them money. So, would they be required to take cash from me, the debtor?

I don't know