HELP! Contract of sale signed, buyer took Fehrzeugbrief!

Hello, need a quick assessment because I now have a bad feeling. Buyer (dealer) bought my car (purchase contract with signatures x2), gave me a down payment and wants to pick up the car tomorrow. We should then re-register this at the registration office together. However, he has already (which I did not notice) taken the vehicle BRIEF with him and the purchase price is not yet fully paid. License plates are still there and the car is in my yard… t “delivery” is not yet completed in the purchase contract, payment of the balance in cash at collection is included. However, the dealer can now simply claim the vehicle belongs to him (purchase contract + vehicle letter) and load it on a trailer and transport it?! Should I report the theft of the vehicle letter to the police immediately? Thank you already!