I guess I am going to jump into this one
I have 2 "adult" children just about the same age as your daughter so I know exactly were you are coming from. Your daughter has acted irresponsible in for the well being of "her" puppy. Do not be a softy and give in to her. The pup is registered to you, microchipped to you, and you have paid the vet bills and taken care of it for months. She has sadly acted irresponsible for the care of her pet for a very long time. I would hands down tell her she was not getting this dog out of your house. She has a lot of growing up to do before she can be considered responsible for another living creature and you have a responsibility to put your foot down and just tell her NO! It is amazing how just because they graduate from HS and go on to college they all of a sudden think they have a right to being considered responsible adults.
Being responsible comes from your own personal actions and not from a birthday.
Just tell her NO, like you did when she was 5 and wanted that pony!
She will live and learn you will see. You might feel really bad now but in your heart you know you are doing the right thing for all involved or you would not have come back to YT for support. You knew when you came here what the answer you would get would be and now take that and run with it! Put you foot down~MOM!