How does the mosque's imam purify his salary from unlawful earnings if he is absent from Dhuhr prayer four times a week due to his work at a private school, knowing that on his day off, he prays the other four prayers at the mosque but is also absent from Dhuhr?

The Imam must perform his duties regularly and be present in the mosque for the required prayers. If he is employed and unable to attend the Dhuhr prayer, he must seek permission from the school or appoint a competent person to lead the people in prayer, while informing the responsible authorities and obtaining the approval of the director of endowments (Awqaf) and the mosque community.

As for the missed prayers during his absence, he must seek God's forgiveness and inform the officials. If they forgive him, then the money is his right. Otherwise, he should pay the equivalent amount to the responsible authority or donate it to the poor if informing the authority is not possible.