Client is requesting a refund for a completed project

[wiredjp]

Hello Everyone,

I was hired by a client two days ago, to complete a fixed-priced contract project on the same day. My client wanted to monitor my progress as I worked on it remotely on his computer.  He wanted to rethink an issue and he decided to extend the deadline for two days. We mutually agreed on this change. However, he part paid me on the first milestone  and he created a second milestone for the final stage of the project. We just successfully completed the project and my client was appreciative of my efforts. He told me that he will pay me for this final stage and close the project. 

So he ended the project and he messaged me to say  he could not figure out how to pay me for the remaining balance. He said that Upwork is requesting a refund from me and he did not want this action to be taken. I am not sure what happened and I told him to contact Upwork Support about this issue. Do any one know how to rectify this issue?

Update: Upwork Support told my client to tell me to decline the refund request and he will be prompted to pay me the remaining balance. 

Thanks and kind regards