My foster daughter is driving me bonkers

Take the dog back! It won't qualify for a service dog, it's your home and if she is in the frame of mind you say already then it's not a good thing. Call law enforcement to stand by when she finds out. Find someone to house the dog for a bit (if you want to avoid the drama a little) and say dog ran off while you were gone. She can't find dog at pound cuz its not there. I don't like the dishonest thing but it's that or straight up tell her the dog goes and call for back up. You beat yourself up enough already, remember and move forward, your heart was in the right place in this.