Is the amount that the merchant returns to my husband, with a fixed increase after one year, considered usury? And what should we do with the extra amount if it is unlawful, knowing that my husband is in urgent need of money and has many debts?

It is not permissible in Mudarabah (profit-sharing partnership) to guarantee the principal capital or a specified, agreed-upon increase. In a legitimate Mudarabah, the agent (Mudarib) does not guarantee the principal capital nor a known profit. Rather, the agreement is based on investing lawful money, and the profit is divided between the two parties according to their agreement. As for losses, they are borne by the capital owner, and the Mudarib loses his effort, unless there was transgression or negligence on his part.

As long as the merchant refused to return the amount before the due date and estimated the increase based on his knowledge, he should be asked to correct the contract to a known percentage of the profit for the capital owner, not a percentage added to the principal capital. However, if the merchant did not mention the profit percentage as a guarantee but rather based on what is generally expected, and he is not guaranteeing the principal capital or any additional profit beyond it, then there is no harm in the transaction.