Is it Islamically permissible for a company that provides fishermen with a cool box for preserving their catch and ice, to stipulate that they sell their catch exclusively to the company at the highest auction price for that day, knowing that the fisherman buys fish from other boats at sea under the same agreement, and then delivers it to the company without knowing the price until the end of the day after the highest auction price has been determined?

The contract in which the insulating company grants the fisherman [something] on condition that he does not sell to anyone else is invalid, whether it is a gift or a sale. For the condition is not valid in a gift, and if it is a sale, it falls under the prohibited "sale with a condition."

As for buying catches from fishermen at sea, with the price being the highest market price, this is not permissible due to the uncertainty of the price. Furthermore, the act of meeting fishermen to buy fish involves "meeting the caravans" (talaqqi al-rukban), which is prohibited.

And the fisherman selling to the company what he caught or bought at the aforementioned price also contains uncertainty. To validate the transaction, the price must be known. If they wish the price to be the highest market price, then the sale should occur after ascertaining the highest price.