I paid a car for 4 years and my FIL isn't letting me keep it...advice?

Edited. Okay someone tell me if I’m mistaken. My SO got a car from the dealership in 2017. My SO and FIL agreed to put it in FILs name. SO made the payments from 2017-2019. My SO gave the car to me and got another car in his name for himself. I’ve made the payments from 2019 until 2mo ago and finally paid it off. Earlier I mentioned to my FIL that the car is going back to the dealership soon to be used as a trade in. He told me no, that’s his car. It’s in his name. Was I wrong to assume the car was to be given to us once it’s paid off? I love my in laws dearly but it infuriates me to think I’ve paid for a car for the last 4 years to not end up owning it and being able to either keep it for myself or use it as a trade in.