What is the ruling on agreeing with a partner to recover the principal in fixed monthly installments, in addition to a fixed monthly amount, given the suspicion of usury (riba)? And what is the ruling on considering the amount paid from the profits as part of the principal to recover the remainder, then calculating the profits for the past period?

If the agreement is to invest money in exchange for a share of the profit, this is a Mudarabah (profit-sharing partnership). Its conditions are that neither the capital nor the profit is guaranteed. If there is a guarantee clause, it is a void Mudarabah, and the owner of the capital is entitled to all the profit, while the worker receives the wage of the like. However, if there is no guarantee clause, then setting a specific duration is permissible. This worker is bound by the contract until the term ends, and he must pay your share of the profit if any is made. He is not allowed to abstain or procrastinate except by mutual agreement to terminate the contract. To ascertain the profit, the capital must be liquidated, and the partner pays you what remains after deducting what you have taken. If you agree for the remaining amount to be a debt, it is impermissible to take an additional sum on it because that would be usury (riba).