Selling car. Give car and send letter only on full payment

Hello dear motor-talk-friends, I want to sell my Audi after a long time. I have a potential buyer at my hand (is a woman and sounds serious on the phone) who wants to buy the vehicle. However, because she has to drive about 350 km and does not like to carry so much cash with her, she wants to make a down payment of 3000 € and keep the vehicle with her. She wants to transfer the remaining amount to me on Monday. remember and keep the letter until the complete sum has been received by me? The vehicle would then be gone, but the letter still with me. Thank you scon in advance for your answers! MfG, Mihd