I have just completed a contract which had two milestones. The first milestone was approved by client and 100 USD of the 270 USD was released. I made the second submission and now asking for release of 170 USD remaining. Now the client says after submission that the quality of both the approved and unapproved submission is not up to the mark and is asking for a complete refund. What should I do. Would be really grateful if you can tell me how to proceed. This is the first time I have got such a request. Thanks in advance.