why does my husband fight everytime he spends money?

He probably does this from stress (why he begins the fight) but as to when he threatens to leave you, this might be a way for him to deal with his own guilt, because I am assuming hes not stupid, he knows it hurts you, but he doesnt want to apologize so he justifies it in his head by hurting you more by saying he will leave you.

The funny thing is, when people post a question about what to do they usually have the answer right in the question. You need to basically tell him exactly what you told us. Tell him that you noticed that whenever he spends money on the car he takes it out on you, and you dont appreciate it. This shows that you noticed it and its clearly stating that you dont like it. If he threatens to leave you, say something like “Do you try to hurt me to cover up your own feelings of guilt about how bad you are treating me right now?” this puts it out in the open and spells it out for him. Sometimes people need the most clear and blunt statements to change. If he doesnt change, then you need to say something like “I dont appreciate how little effort you are putting into my feelings in this relationship. If you cant respect me as a person and not something to take your anger out on then I dont think we can work.” Now this may be an empty threat and you might know that you wouldnt leave him but it is a SUBTLE hint that he might not be the one who leaves. I have a feeling his threats are empty and hes betting that you dont leave him. So turn the tables subtly and it should scare him enough to realize that what he does effects not just you but your relationship.