Removing Counterfeit Pen Marks From Currency

Yesterday I found a 1985 $50 in beautiful shape, except it has a counterfeit pen mark across the reverse. As the first small head $50 in my set, I was quite happy with it but was wondering if there is a way to safely removed the counterfeit pen mark without damaging the note, something similar to using acetone on coins. I'm pretty sure the mark is fresh from work. I also found a binary $20 that has regular marker mark across the top. Curious as to how to safely remove that as well.
from what I've read on google, somethings are acceptable to use like nail polish remover, mild soap and water, hair spray, light grit sand paper, and silly putty. None of these sound very nice to use except the nail polish remover, since it is essentially acetone. Still I'm leery as to use so I figured I'd seek the wisdom of the community. Thanks in advance!

PS. An interesting story I found while searching. Not sure if its true. But I found it interesting if it is.

Cleaning money
Launder (literally) the money with regular clothing. Tie it in a cloth bag first. This is not advised for collectors, but it has been used by the Treasury Department as a way to clean soiled bills. Afterward, the bills were pressed, dried and returned to circulation.