Is returning to a previous mixed-gender work environment, with the intention of earning money to perform the Hajj pilgrimage, considered a sin and does it make the money earned from it unlawful, such that neither charity nor Hajj would be accepted from it?

If the job necessitates mixing with men or being alone with the manager, then it is not permissible, and boredom is not a justification for committing what is forbidden. One must repent and occupy their time with what is religiously beneficial, such as memorizing the Quran, seeking knowledge, and helping those in need. The ruling on the salary depends on the type of work: if it is permissible, then it is lawful; and if it is forbidden, then it is unlawful.