Is it permissible for an employee to take some unused and broken devices from the company to repair and use them personally, in exchange for financial dues that have not been paid to him after he demanded them and the company refused to pay?

The employer must fully pay the worker's due as per their agreement. If the employer denies the worker's right and the worker is unable to obtain it through normal means, the worker may take their right secretly, provided that they do not exceed the agreed-upon amount. This taking is considered a recovery of a right, not a betrayal. However, if the worker believes that the agreed-upon right is less than what they deserve, they are not permitted to take anything in excess of what was agreed upon; otherwise, they would be considered a betrayer.

Therefore, if the worker takes from the malfunctioning equipment what is equivalent to their due from the company, there is no harm, provided that no one is harmed by their action, such as if this equipment is in the possession of another employee and the company demands it from them, so that harm is not removed by inflicting another harm.