Is it impermissible to possess money that was given for the purpose of buying external school books, but these books were not bought, rather they were obtained from the internet, with no intention of buying them despite having the time, and must the money be returned after two years?

The agent must advise their client. If there was no negligence in purchasing the books, then there is no sin. As for the money, it must be returned to the school, whether you were negligent in purchasing the books or not. Keeping it is an withholding of what is due to its owner. However, if you were ignorant of the ruling, we hope that you will not be sinful.