If you know nothing about a deal involving the domain, there is only one conclusion - somebody is trying to steal it.
While winker&winker suggest you do nothing if you don't want to deal, I would suggest you talk to Moniker and make sure that nothing will happen.
It might be useful to report the problem to your local police - I don't know what they can do, but you have a name and address to show them (Nebihu Valmir DE-31167 Bockenem, Ackerstr. 20 is pretty specific, although I'll bet he can go into the "no understand" routine at the drop of a hat). Perhaps they can liase with the cybercrime squad and talk to the Germans.
If you want to tell the German police, you can send email from the link at
BKA Info
That text says "don't use this for emergency situations, report to your local police station", and "don't send any attachments, put everything in your email".
Thieves in general rely on a single instance of a crime being below the £30-ish attention threshold of police, so be sure to mention that your domain is possibly worth £500 or so - that should get their attention.
If I were in your shoes, I would try reporting this attempted theft, although I would not hold out much hope of success. Domain theft is fraud, but don't expect "PC Plod and the Woodentops" to be able to get their heads around it right away. If going to the effort of reporting the matter is a "fool's errand", I apologise.