What is the ruling on a mosque imam leading prayers when he works as a blacksmith and his clothes are stained with grease and dust? And what is the legal evidence for or against the permissibility of his imamate?

The ruling for this man leading the prayer is permissibility, and the dirtiness of his clothes has no effect as long as they are covering [the awrah] and pure. The prayer of those who pray behind him is valid. It is permissible to pray in any pure garment that covers the awrah. The cleanliness and beauty of a garment are among the recommendable acts, not obligations. It is preferable for the worshipper to wear clean clothes for prayer, as Allah Almighty says: {O Children of Adam, take your adornment at every masjid (place of prayer)}. And his being bearded, with no one else among the congregants being suitable, makes him most deserving of leading the prayer.