return the favor
To reciprocate one's actions. You helped me last week, so I'll return the favor and do your filing. If Tiffany's the one who started that rumor about me, I'll just return the favor with a rumor about her.
return the favor
to do a good deed for someone who has done a good deed for you. You helped me last week, so I'll return the favor and help you this week. There is no point in helping Bill He'll never return the favor.