Kvothe borrows from the gaelet "Demon" Devi on several occasions. Each time, she has a minimum amount that she's willing to lend, which is higher than what Kvothe needs.
Each time, Kvothe tries very hard to get her to lend him exactly what he needs, and nothing more.
Why does he do this?
I understand that she charges exorbitant interest rates, and that anything more he borrows from her increases how much it will cost him, but it seems like he's consistently in a position where he's using 99-100% of what she loans him for tuition, and he has nothing left over.
Given how poor he is, it would seem that having extra coin would be beneficial to him. His constant lack of even basic spending money seems to hinder his efforts to earn money.
For example, if he bought some spare outfits, he might be able to find some more respectable opportunities to play for nobles (even if Ambrose has ruined any chances of him finding an actual patron).
Surely there are other opportunities to earn interest on investments someone as smart as Kvothe is could come up with.
Is there a specific reason given for him trying to convince Devi to loan him less, besides trying to avoid incurring the extra interest?