What is the ruling on the prayer of a congregant who prayed an obligatory prayer behind an imam who was praying a supererogatory prayer, and did the imam commit a sin by not informing the congregant that he was praying a supererogatory prayer?

According to scholars, it is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafl (supererogatory) prayer. This is based on the hadith of Mu'adh (may Allah be pleased with him), who used to pray with the Prophet (peace and blessings be upon him) and then return to lead his people in the same prayer. This prayer would be obligatory for them and supererogatory for Mu'adh. Therefore, the questioner did not commit a sin by being someone performing a supererogatory prayer and leading someone performing an obligatory prayer; rather, he was a means for the person performing the obligatory prayer to attain the reward of congregational prayer.