I say, and with God's success: The water in which a person has dipped their foot or hands is only to be used for what has fallen from the limbs or has touched the body, and it is considered little in relation to the rest of the water. Therefore, it is permissible to perform ablution and bathing with it; because the water that has mixed with used water is judged by the predominance of weight, and here the used water is little compared to the rest of the water. Refer to: Al-Labab 1: 18-19, and Al-Hadiyya Al-Ala'iya p39-40, and God knows best.