Is it permissible to forge an official document to reclaim financial dues owed by the employer, knowing that these dues have not been paid despite repeated demands for them?

Lying and cheating are forbidden, but lying is permissible if it is to attain a right or repel an injustice, and it becomes obligatory if the right is obligatory. So, if you have an established right with the company and they refuse to pay it without excuse, and you find no way to reclaim your right except by altering documents, then that is permissible, provided you do not exceed your established right.