Interest Transactions

Q: Are any of the following Interest Transactions?

  1. If a person says "Buy this item for me when you go to the supermarket, it costs R40,but I will give you R50 tomorrow?
  2. Loan me R40, I will give you R50 tomorrow, the extra R10 is a gift?
  3. A person gives me R50 and says "Go buy this item for me, it costs R40 and you can keep the change?
  4. If the person insists on giving the extra amount even when I tell him that it is Interest, what should I do with that money?