Activity/adjective for keep re-mentioning a favor

Let's say a person helps you study for an exam (as a favor). What is the activity/verb that describes that person when he keeps re-mentioning it, as to make you feel that you owe him something.

Is there a one-word verb (or adjective) for it?

For example,

person1: Can you give me a ride to the airport?

person2: Sorry, I'm busy.

person1: But I helped you study for your exam last week.

person2: Ok, I'll give you a ride.

Next week.

person1: Hey, I need some cash. You don't mind giving me some, since I helped you pass that exam two weeks ago.

person2: Ok, stop being such a (the word I'm looking for).

I believe many words could be used in this scenario, such as abusive or manipulative. However, I am looking for a more exact work, which describes the action of constant referencing of a previous favor, to make the other feel obligated to help.

Solution 1:

The best I can come up with is that your friend is

Milking it for all it's worth

Solution 2:

That activity might be called nagging or pestering, and the actor could be called a nag or a pest.

But really it's someone who doesn't understand the definition of the word "favor". A favor is something you do for someone without expecting anything in return.

Solution 3:

A common phrase in American English is call in a favor. This is generally used to indicate someone seeking to be paid back for previous favors given.

Your question seems to suggest a repeated nagging for one or more favors. This is not conveyed by that phrase.

A related phrase, usually used by a person in a power position because of prior favors granted, is I own you, indicating that the person previously favored has an unending obligation to the speaker. Further, the speaker is usually demanding rather than asking for the return benefit.