Not directly related to the allowance but just something to be mindful of if renting.
One semester my son rented the books he could. At the end of the semester when he went to return them they said two were not in returnable condition. They clearly had been used but in my opinion they were not damaged. But they get the last word as to what they will accept, no negotiating.
Well we ended up paying the full price for the books (which oddly higher than the price they were selling them for), plus a restocking fee, plus we had already paid the rental fee.
I was so angry at the time that I didn't realize that we should have just gone and bought new books and then returned those as the rented books. Then we would have saved the cost of the higher full price and the restocking fee.
So while renting can be cheaper I suggest the student be extra careful with them during the year or it will end up costing you much more.
One of the irritating things was the store kept saying that they have big signs tthat make this clear. I stood in the middle of the store and asked that they please point to one. Their response was "Uh, well not now cause this is the end of the semester". To which I replied, "but there are students in this store right now acquiring books for the summer session. What about them?"
This incident still sticks in my craw. Needless to say he doesn't rent books anymore.