Χρέος means debt. However the expression “κάνω χρέη” or more formally “εκτελώ χρέη” has nothing to do about financial debts.
It means “I am doing the job of” or “I am doing the tasks of”. It also implies that I am substituting someone who ought to do this job/task, I am only doing it temporarily as this is not my main task/job.
Εκτελώ χρέη οδηγού. I am doing the job of a driver.
Είμαι οδηγός. I am a driver.