In the given case, Stephan has already spent $4,000 on the restoration of a car. Now there are two options with him:
In the first option, he can sell the car for $5,800 after spending an additional $2,400. So the total amount spent on car is (4000+2400) = $6,400.
Hence the net loss on sale for car shall be (2400+4000)-5800 = $600
In the Second option, he can sell the car for 3,800 without incurring any other cost. Hence the net loss on sale for car shall be 4000-3800 = $200
Hence the loss in lower in the case of the second option. Hence he should keep the car since it wouldn't be rational to spend $6,400 restoring a car and then sell it for only $5,800.