Is money earned from working at a foreign company permissible or forbidden, knowing that I took sick leave from my first job to justify my absence?

The default rule is that money earned from any permissible work is lawful. However, defrauding a national company to obtain sick leave in order to work for a foreign company is forbidden due to the element of forgery, which is prohibited by Allah's saying: (So avoid the impurity of idols and avoid false statement) and by the Prophet's (peace be upon him) saying: "Shall I not inform you of the greatest of the major sins?" (He said it three times.) We said: "Yes, O Messenger of Allah." He said: "Associating partners with Allah, disobedience to parents." He was reclining but then sat up and said: "And indeed, false speech and false witness." He kept repeating it until we said: "We wish he would stop." However, this does not affect the permissibility of your salary from your work at the other company, especially since you are not receiving a salary from the original entity during this period.