You are half right. Forget about the withholding. That comes later. First you figure the tax on your income. The EV credit will reduce the tax liability (not the tax due). It can only reduce the tax to zero. Then your withholding is subtracted. If the tax liability is zero you will get back all your withholding.
Situation A is right.
But situation B is wrong. Your tax is 8,100. Minus the 7,500 EV credit leaves 600 tax. THEN you subtract the 8,400 withholding to give you a 7,800 Refund.
