moved returns the ball on the green

It could mean the shot in question was a tee shot, or an approach played into a green, or any other scenario other than when both balls were on the green prior to making the stroke. Failure to replace the ball that was moved to its original spot; or moving the ball that did the striking (rather than playing it as it lies) results in loss of hole in match play or a 2-stroke penalty in stroke play.
There is no penalty to either player as long as this procedure is followed: The person whose shot struck the ball at rest plays his ball as it lies; the person whose ball was moved returns the ball to its original position.For example, your partner hits a tee shot, then you hit yours, and your ball hits your partner's ball in thefairway. Or your partner is on the green, you hit an approach shot, and your ball hits your partner's on the green.
This is covered in Rule 18-5, Ball at Rest Moved by Another Ball.