Personally, I'd say no. They shouldn't have put them up for adoption if they didnt want to part with them.

I'm going to assume there was no written agreement, but I don't think it matters. Tough luck for them.

Now...if they had decided to keep them while you were on your way over there and informed you of this when you arrived, I would have probably agreed, despite the inconvienience of the drive. But the ferrets have been in your possession at your residence for 12 hrs or more.

Did they give a specific reason for wanting them back? or did they just realize what they had now that it was gone and miss their pets?