To what extent is the fatwa of Sheikh Muhammad Abduh valid concerning the permissibility of pre-determining profit for investors in banks, which was narrated from him as being among the unrestricted public interests (al-masalih al-mursalah)?

If the profit in a Mudarabah (a partnership in which one party provides the capital and the other provides labor) is determined as a fixed amount and not a common percentage, then it is unanimously impermissible, as transmitted by Ibn al-Mundhir. This consensus is a decisive proof, and opposing it is forbidden. This issue is not one of the unrestricted public interests (Masalih Mursalah); rather, there is an established consensus on its prohibition, and consensus is considered a legal proof. Even if it were assumed to be a benefit, it is a benefit that the Shari'ah has abrogated.