Car rate not paid, license plates agreed

Hello dear motor-talker, just a short question. Maybe someone knows something about it. Nen Freund bought a used car in the car trade on installment payment. Now the 6 installments did not pay, the car salesman came to the car now and took the number plates off and moved them in. He only moves them out again when the open installments are paid. Can he do that ??????? buddy can now no longer drive and I also do not always have the time ???? K. A if the right is with the signs Greetings Chuck y