Thank you for the excellent follow up question.
Yes, the arrangement that you have does meet the elements of a contract under Arizona law. However, because she has not delivered on the agreement as you understood it, she also is in breach of the arrangement. This gives you grounds to terminate the agreement. But you must do so in writing.
The amount of money involved is small, so any suit will be in small claims
court. (No attorneys are involved.) It is up to you whether to pay her the additional three hundred dollars now or dispute her version of events. Court costs in small claims court, including service costs are approximately $200.00. Call your local magisterial court clerk and ask them what the costs are for filing a breach of contract
claim. Then you will have all the information you need to decide to take the risk and fight her or pay her now.
Based on your description of the arrangement it seems you have grounds. But only you can decide what is best for you.
I hope this additional information is helpful to you.
(Please do not forget to rate me. It adds nothing additional to your costs, but it helps me greatly. Plus it is good Karma for you. Thank you.)