You should sue her. She breached your verbal contract. Record her voice over the telephone stating the terms of your contract: " That it was $100 but now she wants $ . Blah... Blah..." (She will probably change her story in court) You can use a video recorder (cam corder) and just tape the phone with her saying your agreement.
 
 
It is a standard practice among rescue groups to spay and neuter their animals before adopting them out because they do not want more animals created when there are so many that need good homes.