Also, never trust what people say when they give you a rescue dog.
What a terrible thing to say.
It serves no rescue any purpose to lie about the dog, because in the end, if the dog is not placed in a suitable environment, the rescue will have to take back the dog, and each time a dog goes out into the "field", and is subsequently returned, there is an impact on the dog.
It's possible that you fostered for a rescue of ill repute. Like dog breeders there are good rescues and very poorly run rescues, but don't put us all in the same basket because there is NOTHING farther from the truth - and a dog given to someone else, by someone else isn't a "rescue dog". It's someone trying to dump their problem onto someone else, which is most likely what happened in this case.
On the contrary, in my rescue experience I have met too many people who "think" they have dog skills, and "think" they know about dogs, until the finally get a dog that proves them wrong. This person should consider getting into school to learn some training techniques and should read up on the breed to develop a finer understanding of the dog they adopted - and work with the two existing dogs so that they develop a better sense of social concepts with the new dog.