Is it permissible for an employee to dispose of and sell company-owned items without the owner's knowledge to cover part of his overdue salary, provided he commits to stopping this practice once salaries are paid regularly?

You are not permitted to dispose of the company's property that is in your possession except with the owner's permission and knowledge, because your hand over it is a hand of trust (amanah (definition)), and disposing of it without the owner's permission is a betrayal. However, it is permissible to take from it an amount equivalent to your right if the company owner denies you or procrastinates payment while being wealthy. The delay in paying salaries due to an unforeseen circumstance or incapacity does not permit you to dispose of the trust.