All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Partners in trade or any other type of wealth apart from cattle should pay the Zakah of their money individually once a lunar year has lapsed on it and it has reached the Nisaab even by adding it to cash or other trade goods he owns. A partner should not add his own money to another's so as to reach the Nisaab.
Ibn Qudaamah said: "If they are partners in anything other than cattle, such as gold, silver, trade goods, crops or fruits, then their partnership is of no effect and their ruling is the same as that of non-partners. This is the opinion of most of the scholars. Ahmad holds another view which is that the partnership affects the ruling of Zakah on wealth other than cattle. So, if there is a Nisaab of something whose ownership is shared among them, then Zakah is due on them collectively. This opinion is also held by Is-haaq and Al-Awzaa‘i regarding the Zakah of grains and fruits. The first opinion, however, is the one chosen by Ahmad as he said: "Al-Awzaa‘i said about the Zakah of crops: "If they are partners and the crop reached five Wasqs, then Zakah is due on it.” He deduced this by drawing the analogy of Zakah on sheep. Nevertheless, I do not like the opinion of Al-Awzaa‘i.""
Based on this, once a lunar year has lapsed on the money, each one of you should assess his share of the partnership and, if it reaches the Nisaab which equals approximately 85 grams of pure gold or 595 grams of pure silver, then he should pay its Zakah at the rate of 2.5%.Allaah Knows best.