I'm looking for a yaoi, I think, where there's a demon guy with silver eyes and he ends up losing them to trade for something...but I can't remember. I've been looking for it, forever!

It doesn't sound like this is the one you're looking for, but maybe it can help to 'jog' your memory?

Dokunoaji by Naono Boura

Edit: It turns out that what happened to two of Rakamu's four eyes is only explained in a 4-koma at the end of Yami ni Tooboe, Mune ni Toge...