What is the religious ruling on the aforementioned partnership, how is its Zakat calculated, and is Zakat due for the years in which the money was not at the disposal of the partner?

It is permissible for one person to give money to another on the basis of a partnership, with one of them working, and the profit shared between them according to the capital or as agreed upon. However, a partnership is not valid if a fixed monthly amount is specified for one of the partners; rather, they must share in both profit and loss.

If the Mudarabah (profit-sharing partnership) is invalid, the profit belongs to the owner of the capital, and the worker is entitled to a fair wage (ujrat al-mithl). In the event of an invalid partnership contract of any type, the profit is divided according to the proportion of the capitals, and each partner is entitled to a wage for his work in his partner's share.

If the profit you received is your rightful share, you must pay Zakat on it along with the principal. Otherwise, you are obliged to pay Zakat on your share and return the excess. You pay Zakat on it for one year if it is considered a debt owed by an insolvent person.

As for money deposited in a savings account at an Islamic bank, there is no objection to it. Zakat becomes obligatory on it once it reaches the Nisab (threshold), so you pay Zakat on it every year at the end of the Hawl (lunar year). Any missed Zakat payments must be made up.