Is it permissible to be late for work, or to leave it without a necessary reason, or to be preoccupied with personal matters during working hours, with the intention of forfeiting a portion of the wage in exchange for that? And is it obligatory to forfeit a portion of the salary if an unintentional delay occurs in coming to work?

It is not permissible to be late for work, or to leave before its end without an excuse or permission from the employer. The employee's intention to get rid of part of their salary does not make this permissible for them. If the employee falls short, they must seek the employer's forgiveness or return what corresponds to their shortcoming. However, if the employer or their representative gives permission, then there is no blame on the employee. The basic principle is that the employee should only engage in their job duties, while observing the terms of the contract, what is customarily tolerated, and what the employer has permitted.