What do I do if my soldier has bought an expensive car?

Sometimes you just can't fix stupid and that ship will either have to float or sink on its own.

Failure to manage finances is grounds for separation, and also grounds for a bar to reenlistment. You can't force him to sell his car, even if it is the best idea, but your Commander can require him to stick to a budget or be separated. Talk to your Commander about a bar, sometimes it takes a Soldier seeing that the light at the end of the tunnel is really a train, to snap them back to reality

I've only one thing to add.

Don't make saving his time or career in the army your life goal. SFC Boyd said you can't fix stupid. Yes, try and prevent stupid things, but but at the end of the day PVT's that are fed/sheltered/clothed by the Army will still find a logic in their mind to use their entire remaining expendable pay check for a car payment.

Extend the hand of help, but don't ram it down his throat. Line up your ducks (by way of documenting through counseling statements and sending him to finance) to show you did right as a leader. Otherwise, you're never going to mentally survive as a SSG+ trying to "save" every idiot.

So I dealt with this before with a soldier and it sometimes can come down to how badly you wish to save the soldier and time you can dedicate. So firstly counsel the soldier on the purchase, conduct a separate counseling with a budget for the soldier that way every time the soldier veers from what he agreed you can counsel the soldier, keep an eye out for issues that this can bring into the workplace such as the soldier borrowing money for coworkers, using coworkers to avoid people looking for their money. Often NCOs wait until something big happens and want to chapter the soldier with no supporting documents so ensure you have the supporting documents. If you work a pretty standard schedule you could recommend the soldier pursue a second job or do Uber to bring in cash to fill in the gaps.