I am surprised that people keep repeating over and over that "she is not the right dog for you..."
Isn't that something a responsible pet animal owner determines prior to adopting the animal?? It sounds like the OP took very little time to research what terriers are like. Toffee sounds like every other terrier I have heard of...............it's her owner who is behaving questionably.
Forget the fact that OP is dismissing every suggestion given to her so she might salvage her relationship with this living creature and not return it to possibly perish or be deemed a difficult placement. She already knows she is returning the dog and is looking for reassurance.
Yes, "return" her as if she were a pair of shoes, but don't ask me for sympathy or the stamp of approval. And don't "adopt" another animal! (Did someone actually suggest OP might not be a "dog person" but a "cat person" instead?) We are talking about unwanted animals, right? Maybe OP is a reptile person and should go in that direction or a fish person? Maybe run the whole gamut of options and treat the shelter like a freakin'
Nordstrom.
Sheesh. I need a cocktail.