Vehicle sold for 7 months but still not re-registered…

Hello, I know this is once again a case where you can only shake your head. I sold a horse truck registered in May 2010. I also made a purchase contract. The buyer also gives me an extremely serious impression. But since I had a lot to worry about, I didn’t take care of it any further and I assumed that the truck was also immediately re-registered. Yesterday the tax office booked off the taxes again. Then I am directly at risk to the buyer. See that the truck was standing there, and was still registered on me. The face color swarms very quickly on when he registered who had ringed there. From the conversation then came out that he wanted to sell the truck again. I then offered him to buy the truck back, minus the insurance and taxes that I had already paid. I found this more than fair. The buyer also agreed, but called me today and said that he did not do that. n wants, unless I’m ready to add 300,-euro! I told him that it’s pure coming from me, and I find it an impudence that he is now pushing up the price after my very kulant offer! I know that I have to report the whole thing to the road traffic office. My question is now whether I can still show the buyer because of fraud/insurance/tax evasion or whatever? Thank you for your answers….