
Although strawberries aren't toxic for dogs, it's possible that whatever he's eaten has disagreed with him - or it's possible that it's the after-effects of the vaccine. Whichever it is, he shouldn't go to his new home until he's better; in itself that's enough stress to the system to cause the runs, so if he's already not quite right he could end up being quite seriously ill. It looks like he'll be with you for another few days - disappointing to the new owner, but it's the puppy's welfare that's important.