Is the salary earned from a music teacher's job (160 EGP), without teaching music to students and utilizing the classes for other permissible activities (such as doing homework, studying, answering religious questions, reading and memorizing the Qur'an), lawful?

Falsifying certificates is forbidden (haram) in Islamic law. Therefore, the questioner should fear Allah and strive to obtain another job with a valid certificate. Allah Almighty says: "And whoever fears Allah - He will make for him a way out And will provide for him from where he does not expect." The fact that music is forbidden does not justify falsifying a certificate to work with it. Rather, one must leave this job and search for another, and whoever gives up something for the sake of Allah, Allah will compensate him with something better.