return a favour BrE
return a favor AmE
return the favour BrE
return the favor AmE

But don't think I'm not going to try to return the favor next time.

Maybe if she gave him something, he would return the favor.

He had done so much for me, I would be happy to return the favor.

"And I see no reason why we should not return the favor."

But just so you know, we're going to return the favor.

Now will they please return the favor and come back home?

"We must figure out a way to return the favor."

"One day I may return the favor you did me."

I raised my glass, more than happy to return the favor.

It was nice, for a change, to be able to return the favor.