I say, and with God's success: The impurity in the shoe, if it has a substance: it may be dry or wet: If it is dry: it is purified by rubbing; because the trace of the impurity is removed. And if it is wet: it is purified by rubbing on the condition that it is done thoroughly so that no smell or color of the impurity remains according to the fatwa. Refer to: Al-Waqayah and its explanation by Sadr al-Shari'ah, p. 130, and Naf' al-Mufti, p. 137, and God knows best.