Car sold privately, now the Kaeufer wants to return 100E

Hi all, I sold my old car last week (for about 500 euros), the bank wanted to have the car fast, and also did not want to sign a contract etc. but then we did it. He said that he is a mechanic and looked at the car (motor also) and checked everything and was satisfied with it. I wanted to sell it for 650 but then I sold it for less than 500. I told him the (known by me) maengel, but after 3-4 days he did I’m a foreigner and I never sold a car, I don’t know how it works. The car is without TUV and my mechanic didn’t tell me that the gearbox was kaput, that’s why I didn’t know it (even because the car always went well) and honestly I don’t know if it’s really kaput or does the buyer just want money from me. Do you have any tips? I don’t want to give him any money, also because he can always say later “Oh that’s also kaput I want 100Euro back” etc. I apologize for the mistakes I’ve made, hope you understand my words. Thank you in advance