Tell them, you belong to a group and a lot of people are working on trying to get him into a rescue or foster home. Could they please give him some extra time, so we can at least get answers from the rescues that have been contacted and also see if we have any way of finding a foster home thru the group.
That should be all you have to say, and also ask lots and lots of questions about disposition, previous situation, have they seen any difficulties at all with him, how is he with dogs, cats, kids, well just everything you can think of that will make it easier for people to know if they can foster this dog or not.
Good Luck
Marit