How is the partner's share of the speculative capital (mudarabah) to be purified with zakat?

For Zakat to be obligatory, it is a condition that each partner's share reaches the nisab. If the money is shared such that each partner's share does not reach the nisab, then Zakat is not obligatory on either of them. If your father contributed the capital and you contributed the labor, then the capital belongs to your father, and Zakat is obligatory upon him. However, if your father transferred ownership of this money to you both, then each one of you should add his share in the company to his other savings, and Zakat becomes obligatory if the total amount of money reaches the nisab.