As-salaamu 'alaykum, My brother is the owner of a small enterprise that take part in Government bidding for the supply of Medical, Surgical and Lab Items. Please note that the way his enterprise works is that he takes part in Bidding for the yearly, monthly or occasional bulk supply. After winning the bid he makes a contracts with customers for the supply according to the government rules he has to put certain amount in bank as a security deposit which remains in account throughout the time period of the contracts. He buys the items from vendor or manufacturer and brings items to his warehouse from where he supplies to his customers, and some of items remains among stock. His customers pay him wholesome amount after completion of supply contracts. Last year he had number of supply contracts but he didn't save any money rather he invested his profit again his business. Right now he has ongoing supply orders/contract, Security deposit/checks and remaining stock.
My question is does he should include the amount that's not back yet from the contracts and security checks. How Should he pay zakaah?
I hope that you can tell me the right way of paying zakaah, because I am confused about his situation. I pray that Allaah will guide us to the right way.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The sums of money that your brother has not yet received take the ruling of debts owed to him. If he believes that they will most likely be paid, then, if he wishes, he may pay the Zakaah due on them along with the rest of his wealth, or, if he wishes, he may wait and pay Zakaah on them for the past years when he receives them. To pay Zakaah, he calculates his property that is liable for Zakaah, adds to it the money he loaned to other people, and deducts the debts he owes to people and then pays the Zakaah on the result by paying one-quarter of one-tenth (2.5 %) of it. Or he may wait until he receives these outstanding payments and pay their due Zakaah for the past years. For more benefit, please refer to Fatwa 82524.

Allah Knows best.