Does the transaction become forbidden if the merchant gives a non-fixed monthly percentage to avoid falling into usury, and what is the solution?

There is no harm in the profit ratio varying among shareholders according to the variation in their capital. If it is agreed in the contract that the worker (mudarib) has a known percentage of the profit (such as 70% or 80%), and the remainder is divided among the partners according to their capital ratios, then there is no harm in that. The consideration is what is agreed upon in the contract regarding the method of profit distribution, even if there is a disparity. A known percentage of the profit must be specified for the trader (mudarib), not from the capital. If the contract is valid and meets the conditions, there is no harm in continuing with it. However, if the contract is invalid, then the capital and its profit belong to its owners, and the mudarib receives a compensation for his work (ajr al-mithl).