Question on Diamond

Hi, I would like to sell a diamond. My friend purchased it 1 yr ago from Single Stone in Los Angeles. The stone is cushion cut, 3.87 ct. My friend paid $20,000. which included a simple platinum setting. The engagement fell apart and he needed $ so I purhased it. I do not have an appraisal so I assume I need to start by getting one. I do have the receipt from Single Stone. Thank you for your opinion