I'm going to demand a rescue take a foster dog back
Do they usually accept returns of live animals?
Taking away dogs from their foster parents are quite common.
If you feel that you cannot take care of the dog, it is okay to give it back. Foster situations are only temporary anyway.
What should I do about it then?
I think that you should talk to them then. if you think that you cannot give the dog a proper home then you should try to return it so that it is safe.