My Bil Sold My Sisters Engagement Ring. Can I Take Him To Court?

I originally posted somewhere else, but I need advice on if I have a case here.

A couple months ago, my sister lost her engagement ring in my car. We searched until 3AM and couldn’t find it. Recently, the car because unreliable and expensive for a college girl, so I had to sell it quickly to my BIL. My sister gave her word that it was okay, and that she hopes BIL can find it when he goes to restore the floorboard/carpets. I have in writing (text messages) that if he finds the engagement ring I want it back because it’s my sisters. He agreed (again through text) and said he would let me know. Well, I found out on Friday night that he found the ring, but sold it for $3k on FaceBook Market Place. I demanded that he find the guy he sold it to and get it back to me. He said the guy had ‘deactivated’ his account, therefore, gone. I said he needed to fork over the $3k. He refused. So I said at least pay up $450, which is how much my sisters fiancé spent to set the ring (he already had my sisters mothers diamonds [around 1.3ct] and moonstone). He again declined. Can I take him to small claims court or sue him? Where do I go from here?