How does one purify with zakat the money earned from his previous job, for which a specific monthly wage was not agreed upon until after three years, and now he receives his wage regularly, taking into account the purchase of a truck in installments in partnership with a friend and borrowing a sum of money that has not been fully repaid?

For the lease to be valid, the rent must be known. If this condition is not met, the lease is void, and the lessor is entitled to the customary rent (اجرة المثل).

If the agreed-upon rent is the customary rent, then Zakat (definition) is due on all of it as soon as it reaches the nisab (definition) (minimum threshold). However, if it is more than the customary rent, Zakat is not due on the excess because it is a donation. If it is less and the lessee refrains from paying, then there is no Zakat except on what has been received, and the debt received for past years is subject to Zakat, according to the preponderant opinion.

The Zakat on the customary rent paid in installments is due upon the completion of a hawl (lunar year) for each installment. For precaution, it is permissible to pay Zakat on the entire rent for all past years upon the completion of a hawl for the first nisab.

After agreeing on a specified rent, its Zakat becomes due if it reaches the nisab upon the completion of a hawl for each installment, or upon the completion of a hawl for the principal amount for precaution.

Regarding deducting debts from wealth, it is more cautious and absolves one's liability (الأبرأ للذمة) not to deduct debts and to pay Zakat on all wealth if it reaches the nisab.