How should he pay zakaah on a car if it stayed in the showroom for three years and its price has fallen?

If a person buys a car with
the intention of selling it and making a profit, he must pay zakaah on it
every time a year passes. He should work out its value on that day, i.e.,
its market value on the day when the year has passed, and pay one-quarter of
one-tenth (2.5%), whether the value has increased or decreased. 

Shaykh Ibn ‘Uthaymeen (may
Allaah have mercy on him) was asked: Someone bought some land with the
intention of dealing in it, but it remained in his possession for a long
time. Is zakaah due on it? 

He replied: 

If a person buys land with
the intention of dealing in it, then he must pay zakaah on it every year,
whether its value increases or decreases, and whether it sells or not. He
should work out its value every year, then if he has money he may pay the
zakaah from the money he has; if he does not have money he should keep a
record of the zakaah that it due every year, year by year, and when  he
sells it he must pay zakaah on the past period. End quote from Liqa’
al-Baab il-Maftooh (15/12). 

Based on this, the owner of
the car should have looked at its price at the end of every year and paid
zakaah on it. If he did not have any money, he should have kept a record of
it and paid it when he sold the car. 

It should be noted that the
year with regard to trade goods is the year from when one acquired the
wealth with which he bought them, if it reached the nisaab (minimum
threshold). The year for the car in this case does not start from the time
when he bought it, but from the time when he acquired the money with which
he bought it. 

What the owner of the car
must do now is work out what the value of the car was at the end of each
year, with the help of those who have experience in the market, then pay the
zakaah. 

And Allaah knows best.