Motorcycle sale – registered, without license plates – registration/registration request

So I’m trying briefly: My 125s will be picked up tomorrow. It’s still registered, I keep the registration number because it’s a desired number. The buyer gets everything that belongs to it, of course. In a week the new machine comes – this should then be registered or re-registered on my desired number of 125s. Now my questions are: Can the buyer of my 125s unsubscribe with the papers, but without the registration number? Or can he then only re-allow it? The buyer doesn’t come from the same city. Otherwise it would be easy… And if I didn’t WE would just sign them off for safety. If my planning didn’t go, what would you suggest?!? Should he take my license plate and sign it off? (signs back to me by post) Or should I give him the machine without papers (I’ll go off on Monday). Or is there any other way? Thank you!