Is there another way you can phrase the idea of “elaborate classes”?
Otherwise, if a teacher asked for a book from a school, saying he needed it to teach a class, and he gave it to a student to help them in the class, then that doesn’t seem to be a problem on the surface. In some school districts, books are replaced periodically after x years of use, and they buy the latest-and-greatest version from the academic publishers. So those older books end up getting thrown in the trash, sold at book sales, given out to whoever wants to grab a copy, etc…
He might not have been specific about his exact intentions-- “I need this book to teach class” vs “I have a student who needs this book” --but ultimately, it sounds like the book was destined for the dumpster, he asked for permission to take it, and now he’s passed it on to you to make use of.