Is the issue at the buyer or sellers end ?

If its the sellers end and you have agreed to hand over the car by a certain date(today). yet its not happening till tomorrow then id say the deposit should probably be handed over.

If its the buyers side, tough luck and keep the deposit.

I normally have it all in writing to say so and so has agreed to buy car, with a deposit of $$ with final payment happening on so and so date. Make it as air tight as possible.

Thats normally the time you weed out the time wasters.