What is the ruling on soap upon which an impurity has fallen?

If impurity comes into contact with soap, then if the soap is solid and has a firmness that prevents the impurity from spreading to its other parts, it is sufficient to remove the part that was affected by the impurity, and the rest remains pure. However, if it is liquid such that the impurity spreads to its other parts, then it does not become pure, and it is not permissible to use it. This is as stated in "Al-Mughni" by Ibn Qudamah, who cites the Hadith (definition) of the mouse that falls into clarified butter (ghee): if the butter is solid, what is around the mouse is removed; but if it is liquid, it is left.