Yes, you may have been duped a bit by her giving you the b.s weight guarantee but you should have known that wasn't possible in the first place. The price was great either way and you have to take your share of the responsibility here. You didn't do your research and you got bent. Lesson learned. (hopefully)
I really hope you don't take this out on the pup and that you can love her and take care of her forever. If not, please rehome her, she doesn't deserve anything but the best!
I guess I'm a little shocked. Considering the OP has 3 other yorkies, I would think she would know that there is no such thing as a "teacup" just as there is no guarantee of size.