Is it permissible for me to purchase my partner's share of goods that have not yet been acquired, knowing that they are still at sea? And does this fall under the category of a sale involving uncertainty (Bay' al-Jahalah)?

Your partner is not permitted to sell his share of the ghee before taking possession of it, due to the Prophet's (peace and blessings be upon him) prohibition of selling food before taking possession of it, and because scholars have unanimously agreed upon this.

Furthermore, dealing with the original seller's agent in the manner described is impermissible due to two prohibitions: 1. The unknown nature of his commission for delivering and selling the goods. 2. Appointing the seller as an agent for selling the ghee, which is not permissible.