How is money stolen from joint business funds calculated, and does each person pay zakat on his money, or does the speculator pay zakat on all of it?

If money is stolen due to your negligence, you are liable for compensation. If it is stolen without any negligence on your part, then you are not liable for compensation, as you are considered a trustee. Stolen money is treated as if it no longer exists, and the partners remain partners in the remaining capital. The zakat (definition) on Mudarabah (definition) wealth is incumbent upon the capital owners. Each of them must pay zakat annually on their share, including their profit, provided that a year has passed on the Mudarabah capital. Your share of the profit is not subject to zakat now; rather, you will start a new year (for zakat calculation) when it is distributed to you.