If your share reaches the nisaab or minimum threshold for grains, then you must pay zakaah on it. If he paid zakaah on the whole amount before giving you your share, then that is sufficient. But if he gives you your share then he pays zakaah on his share only, then you must pay zakaah on your share if it reaches the nisaab for grains, because you own it at the time when zakaah becomes obligatory – which is when the grain ripens. The nisaab of grains is three hundred saa’s of the Prophet’s saa’. End quote.