By default, I consider any created as commission content as permitted to use in my future public releases. If you are against of that, just tell me about it, in most of cases that would be free, if commission is advanced and big (car pack with few cars, or vehicle converted from other game), I may ask a little bonus for that.
If I do specific variants of cars, almost in all cases except of lore-friendly cars, I provide finished car in LOCKED condition. Getting of UNLOCKED variant usually not available, except of cases when vehicle is available as UNLOCKED for both of us from start. To be fully clear, you pay for modifying of models for you, not for models themselves.