What is the ruling on the prayer of one who prayed Asr with the congregation with the intention of Maghrib, and did not remember until just before the Imam's bowing in the first rak'ah, and did not repeat the prayer after one of the students of knowledge issued a fatwa to him that it was valid?

If someone prayed 'Asr with the intention of Maghrib, their prayer is not valid, because intention is a condition for the prayer's validity. What was obligatory was to exit the prayer and repeat it with the intention of 'Asr and the opening Takbir. For, transitioning from the intention of one prayer to another merely by intention is not valid. Indeed, whoever intends to transition from one obligatory prayer to another, both prayers become nullified. Therefore, the 'Asr prayer must be repeated.