Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
I believe that Zakah is on the net worth of business. Therefore, on the day of your Zakah you calculate your net worth and make some adjustments that include: pricing assets and inventory at market price on that day, removing accounting adjustment assets such as prepaid expenses, etc.
You then calculate Zakah at 2.5% on the adjusted balance of net worth. If you don’t have sufficient cash at that moment, you may delay all or a part of the amount due until you have sufficient liquidity without sin because this is a Shari`ah valid excuse.
But you must remember that any non-paid amount of due Zakah, for any reason, remains a debt on you until you pay it out.
Allah Almighty knows best.