What would you call someone who has an urge to repay debts / favours?

A conscientious person would feel guilty if he/she didn't repay a small debt or favour through forgetfulness.

(vocabulary.com)

• very careful about doing what you are supposed to do : concerned with doing something correctly (MW)

scrupulous - having scruples, or moral or ethical standards; having or showing a strict regard for what one considers right; principled _{^{(dictionary.reference.com/)}}

In some contexts, scrupulous can have significantly positive or negative connotations, but scrupulous about repaying debts, for example, can be simply a "value-neutral" description of how someone acts.

Mirroring OP's exact context, Friends And Heroes: The Balkan Trilogy 3, (Olivia Manning, 2011)...

He was ... scrupulous in paying his round. He was equally scrupulous in seeing himself repaid.

(In that context, his scrupulousness is clearly seen as a "social asset" by the speaker).

In the context you provide -- the repayment of debt or favors -- you might call this person:

• duty-bound
• creditworthy
• principled
• responsible

Obligation (obliged) is the word for someone who is bound to follow a certain course by law or custom. Your rent payment is an obligation, whether or not you remember to pay it, and whether or not you feel guilty for forgetting to pay it.

Beholden is someone who is obligated to follow a course by morals, and might better fit the person and situation you describe.

I believe that returning favours is, at least to some extent, a question of politeness. So, perhaps you could describe the person in question as 'courteous' or 'well mannered'?

Hope this helps