What is meant by "the removal of the reason for the concession" in the context of pardoning impurities? And does the mentioned obligation to wash them mean a fard (obligatory duty) that invalidates the prayer if left undone, or is it merely recommended (mustahabb)?

The aforementioned text is from the book "Manh Al-Jalil Sharh Mukhtasar Khalil" by the scholar Alish Al-Maliki. It pertains to impurities that are pardoned due to the difficulty of their removal, such as the impurity of those with continuous ritual impurity (ahdath da'imah). If this difficulty ceases (e.g., a boil heals or the continuous ritual impurity stops), the reason for the pardon is removed, and the impurity reverts to its original ruling, making its washing obligatory or sunnah (definition). This refers to the disagreement within the Maliki school regarding the obligation or sunnah status of removing impurity.