Dollar bills are incredibly dirty with germs/bacteria, etc. You can try and scrape it off with a razor blade, but in terms of harm reduction it is probably better to just let it go and not bother. I can't imagine you would even get very much powder off of it anyways. Soaking it in water would in theory work but it would be INCREDIBLY un-sterile and I strongly recommend not doing it.
Probably better to just cut your losses.. It is probably all cut, anyways. In the future use a hollowed out pen or a straw, so if you accumulate a lot of residue you can just cut it open and scrape, and it won't be germ infested like a dollar bill is.