Is it permissible to buy and sell fodder with partial payment and to sign blank checks to guarantee rights, and what is the legitimate way to guarantee rights when not possessing large capital, and is it permissible to sell fodder at a fixed price despite changing prices?

The ruling on the transaction depends on the agreement between the two parties. If the agreement is to sell a known and specified quantity of fodder, to be left with the merchant for the friend to take as needed, and payment is made in installments, then this is permissible.

However, if the agreement is for a quantity to be taken over periods with the price paid in installments, this is not permissible because it is a Salam sale that does not meet its Sharia (definition) conditions.

If the agreement is to purchase a large quantity to be taken in parts, and the sale contract is concluded for each part at the time it is taken, then this is permissible. There is no objection to delaying the payment or part of it for a known period.

There is no harm in your friend selling it before settling the remaining price with the merchant, as the condition is that the sold item must have been owned and possessed.

There is no objection to documenting debt with checks, but caution must be exercised with blank checks due to the risks involved.

It is not permissible to increase the debt if the debtor delays payment, as this is forbidden usury (riba). There is no harm in selling at a fixed price even if market prices change.