How is zakat paid on money from selling cars in installments, some of which is collectible and some of which is doubtful?

Debts are of two types: The first is a debt owed by a solvent person who pays upon demand; this is subject to zakat annually at a rate of 2.5%. The second is a debt owed by a poor person or a procrastinator; this is not subject to zakat until one year after its recovery. Sheikh Ibn Uthaymeen was asked about the zakat on debts, and he replied that it is obligatory to count cash, trade assets, and debts owed by people, and to pay zakat on all of them. It is permissible to pay zakat on debts every year, or to delay its payment until they are recovered, and then pay zakat for the past years, unless the debtor is in straitened circumstances, in which case zakat is paid for one year only, which is the year of its recovery.