I sold a domain for $12K. I transfer the domain from Yahoo Domain to this company's GoDaddy account. They did not change the registrant info so on the WhoIs, I'm still there! They were supposed to pay me in 12 monthly payment but after 2 months with no payment received, I sue and we are in court January 3rd... They now claim that they never received the domain! I do have the Domain Transfer confirmation from Yahoo. They shut down the site last month... How can I prove they own the domain? Merci!
This is why one should give these things a long and hard thought, especially when dealing with parties outside of your jurisdiction. All I can suggest at this point is to take a screenshot of the domain name's current WHOIS since you say it still bears your registrant info. Don't expect it to bolster your case, though, since WHOIS isn't necessarily accurate. In any case, let your lawyer handle this. If your current one isn't competent enough, then get one who is.