Is it permissible to purchase a device that was taken without the permission of the official in charge, knowing that its value has decreased? Should it be purchased at its current price, or at its current price with the difference from its original value being returned to the company?

What you have done is a sin for which repentance is obligatory, because it is an encroachment upon the property of others. The Prophet, peace and blessings be upon him, said: "Every Muslim's blood, property, and honor are sacred to another Muslim." What is incumbent upon you now is to purchase a device with the same specifications as the first device, and you are not required to pay any additional amount beyond that.