Answered by Shaykh Rami Nsour
Question: Is it permissible to use soap to make ghusl in the Maliki school considering its strictness on rubbing the whole body with the hand?
Answer: You must not use soap during a ghusl in place of the obligation of rubbing the body. In the Maliki madhab, one of the conditions of wudu and ghusl is rubbing (dalk) which is defined as “passing one limb over the other” but it does not have to be with the hand alone [Dusuqi, Hashiya al Sharh al Kabir]. You can technically use your forearm to rub your other arm, for example. You can also use a rope or the like in the case of areas that are hard to reach [Ibnr Ashir, al-Murshid al-Muin].
The issue with soap is that it changes the water and one must use water in its natural form (mutlaq). If the water reaches the body in a pure form but changes after it has made contact with the skin, like if there is soap on the body, then that is excused [Dusuqi, Hashiya al-Sharh al-Kabir]. The best thing is to avoid soap during a ghusl to ensure that all water reaches the body in a pure form (mutlaq). If one wants to use soap before or after the ghusl to remove filth, then there is no harm in that.
And Allah knows best.