How is the Zakat calculated on a company's funds when there are post-dated checks and expenses due for payment after the Zakat year has passed, bearing in mind that the customer has paid the value of the service in advance for a full year?

Zakat must be paid on installments due, whether they are received during the Zakat year or after it, either by collecting the Zakat directly or upon receiving each installment, while calculating the Zakat for all past years. As for uncollectible installments (such as debts from an insolvent person or a defaulter), there is no Zakat on them until they are received, and they are then subject to Zakat for one year. As for expenses that will be disbursed after the time of Zakat payment for providing services, they are not deducted from the Zakat base.