Is what was taken from the aforementioned company's remnants, which transferred its possession to the building's owner with the intention of leaving them for the new tenant, considered lawful or unlawful?

If the ownership of electronic devices transfers to the building owner, then it is not permissible to take anything from them except with his permission. What was taken with the permission of the first owner (the company owner) before the transfer of ownership is lawful. However, what was taken after the transfer of ownership must be returned, unless the building owner gives permission.