Is it permissible for the Imam to clean the mosque himself and receive the designated wage for that, knowing that his job title is Imam and not cleaner?

If someone donates money for cleaning the mosque and does not designate a specific person, then there is no harm for the one who performs this work to take the money. This is because this money is not a wage for the work, but rather a provision and an aid to obedience. It has been taken from the public treasury or from funds endowed for charitable acts. It is permissible to take provision for works whose benefit extends to others, such as teaching and calling the Athan, for the intent behind it is to assist in obedience.