Is it permissible for the sister's husband to take a 10% share of the capital and profits in exchange for his negotiation to recover it, considering that the contract was originally signed, and is this percentage considered large given his refusal of the 5% offered to him?

It is permissible to take money or a percentage in exchange for collecting outstanding funds, but with the proviso of avoiding greed and exploitation of need. Some jurists, including the Hanbalis, have permitted the commission to be a percentage or a common share of the money, likening it to Mudarabah, such as collecting money for a portion of it. There is no harm in taking 10% for collecting money if mutually agreed upon. It is permissible to take a fee for collecting non-interest-bearing debts. As for interest-bearing debts, it is not permissible to collect them nor to take a fee for doing so. The amount of the fee and the time of its collection are determined by mutual agreement between the parties.