Is it permissible to skip congregational prayer in the mosque for the purpose of buying fish at auction, knowing that skipping it leads to performing the prayer congregationally in the market after its time in the mosque, and that this market is rife with immorality and debauchery, and there is no designated place for prayer therein?

It is obligatory to attend the Fajr prayer and other obligatory prayers in the mosque, unless the worshiper fears harm to his essential livelihood. If a Muslim prays in congregation in the marketplace, there is flexibility in the matter as long as the place is clean, and the clamor or indecency of speech does not affect the validity of the prayer.