Do you deserve negative feedback?
If the client leaves (or threatens to leave) negative feedback because she is trying to get the work for free, then that is feedback manipulation. Of course that is wrong, and that is a violation of Upwork TOS.
But even though something is wrong and immoral, it may still be possible for a client to use the Upwork user interface to do it.
You can not report a client for feedback manipulation if she has not actually said anything about that yet. At this point, you are only speculating that the client will leave feedback if you don't issue a refund.
One thing to remember is that we work for money, not for stars.
$600 is a lot of money. You worked for it. You deserve to be paid for your work.
If it was me, I would not want to do any more work for this client. I believe the client is acting irrationally. I believe that the client is trying to steal money from the freelancer.
If it was me personally, I would keep in mind that I have two goals: maximize my earnings from this project, and minimize and negative impact done to JSS.
I don't have all knowledge of this situation and I definitely have not heard the client's side of things. Based on what I have read in the thread, here is what I would do:
Send a message to the client:
"There are two separate questions to be answered. The first question is if you want me to do additional work on the project. If you want me to do more work on the project, then you can tell me what you would like done. Then I will tell you what the cost would be, and you can fund a new escrow payment for that work. If I do not hear from you within 24 hours, I will assume that you require no further work from me, and I will close the contract. The other question is about a refund. I can issue a refund to you, but that will need to be done after the contract is closed."
Then:
If the client wants more work, she will need to to pay you the money you ask for. Which might be a lot.
If the client wants a refund, she needs to let the contract be closed. And she needs to leave feedback before you will do anything. If she wants money, she should understand that she is not going to get refund money from you voluntarily if she is abusive.
If she is nice, and respectful, and honest, including in the way that she leaves feedback, then you SHOULD give her an appropriate refund, in an amount of your choosing. You are the freelancer who did this work and you are the only person who can decide what an appropriate refund amount is. Is it 5%? More? Less? That is for you to decide.
