Is it permissible for a person to pay the price of feed for fish farming, and for the owner of the feed to transport it to the farm, on condition that the payer receives an additional sum for each ton after the harvest period, in addition to the original amount paid?

What you mentioned has two possibilities: Either the transaction is a loan with an increase, which is forbidden usury (riba), or it is a murabahah, where your friend buys the feed and it enters into his possession, then he sells it to you for a profit, and there is no harm in this.