What is the ruling on praying behind an Imam who pronounces the letter "ṣād" in Al-Fatiha as if it were a "shīn"?

If a person has a slight lisp that does not harm his prayer as an imam, then his prayer is valid. However, if someone substitutes one letter for another in Al-Fatihah, despite being able to pronounce it correctly, then his prayer and the prayer of those behind him are invalid. But if the substitution is due to a lisp, accent, or foreign tongue, and he is unable to correct his pronunciation, then his prayer is valid. Scholars have differed regarding his imamate, and the most preponderant opinion is that it is valid.

If the imam pronounces the letter "ṣād" like "shīn" while still producing the essence of the letter, then his prayer and the prayer of those behind him are valid. However, if he purely substitutes "ṣād" with "shīn" despite being able to pronounce it correctly, then his prayer and the prayer of those behind him are invalid. But if the substitution is due to an accent or foreign tongue, then his prayer is valid, and similarly, the prayer of those behind him is valid according to the most apparent opinion. Nevertheless, he should not lead people in prayer if there is someone more proficient in recitation available.