If you try this, make sure the back side of the slot is slightly lower than the front(which faces the fretboard). If the slot is level, it will create a dead sounding open string. It can sound like a sitar(you know, one of those eastern indian sizzly stringed instruments).This angleing of the slot may help your string hold in the nut better. If you drop the front of the slot when it doesn't need to be lower, it's gonna buzz on open strings.
If you lower the slot too much you can fill it with super glue and re-cut it.
It definitly takes practice to properly cut nut slots. It can be real frustrating till you get the hang of it. The correct files to do this are rounded on the bottom. It doesn't really work well to use a square shaped file. Not that it can't be done, it just would take some extra care.
This is a real easy fix for someone with experience. Cheap too I would imagine. Probably worth making a few calls to some shops. Someone who was smart would offer to do it for nothing, and gain a future customer. Not to mention referals.