AITA for punishing my son after he went behind my back to get the car I wanted him to have?

I recently traded in my 4Runner for a new car. My 4Runner was in excellent condition with very low mileage. Before that I was saving it for my twin daughters (F17) when they turn 18 in a few months. The plan was for one of them to have the 4Runner and the other to have my wife's SUV that she just also traded in for a new car.<br><br>My son (M20) has his own car and has always been jealous of my 4Runner. He has repeatedly tried to convince us to let him have the Runner when the girls turn 18 but we've told him no because the girls have waited for so long for it. Last week I got a call from the dealership about the papers and stuff for the trade in, and when I asked which car they confirmed it was the 4Runner. I was confused because we had both made it clear to our son he could not have it and thought he had changed his mind. I go to the dealership and sure enough they tell me that my son came in about a month ago with the title, car, registration, etc. and had it traded in with no questions asked. I went home and went absolutely ape shit on him. I yelled, I screamed, I grounded him, and I punished him. After everything was said and done and he had gone to his room I realized that my son is an adult and I dont actually have the right to punish him. After the whole incident had passed and I had calmed down more I realized I probably went overboard with the punishment. But at the same time he went behind my back and did something I specifically told him not to do like 5 different times. AITA?!