Is it obligatory to return an electrical cable that was taken under the assumption that it belonged to the questioner, and after realizing it was not, he procrastinated in returning it for three years, along with paying the rental value for its use, or is it sufficient to just return it to the school?

The usurped item must be returned to its owners along with payment for any decrease in its value due to use. If it is an item that would typically be rented, the usurper is liable for its rental value (ujrat al-mithl) for the duration it remained in his possession. If the usurped item is present, it must be returned. If the item's value has decreased, it is returned along with payment for the decrease in value. If the item is lost, its equivalent must be paid. In the event of a decrease in the usurped item's market price, the usurper guarantees this decrease in addition to compensation (arsh) for any deficiency resulting from use. The rental value (ujrat al-mithl) is estimated by experts.