Since you agreed to give her a female puppy, 'OR' pay her $750.00 for the stud fee, I 'think' you may have the right to pay her the stud fee now, and keep the puppy.
I would suggest you call the owner of the stud and remind her of that, and tell her you will be paying her the fee.
I can tell you really care about this puppy, and I hope you will let us know that everything worked out and you kept her.
I suspect you will find her a better home than the one the owner of the stud, chose.