Personally I don't see a problem with this, as long as its done in the right way.

By the right way I mean as long as the dog hasn't been returned due to issues on his part, if its because the new owners feel they cannot cope with a pup or they have been made homeless etc I'd rather have it back with me than bundled off to a rescue or passed on to someone else.

If after a few weeks/months the dog is doing fine and a new home comes along I don't see a problem with letting him go again; I have to say I would be extremely picky about who he went to a second time - a family member or close friend would be ideal.