yes you can salvage a mod out of an item with a damage mod on it, just not a collector's item -- which is probably what you tried. If you find the right buyer i supposed you could get a nice price for that bow since people like to have a 15>50, then a 20<50, when they get low
salvage and sell the tring for 50k+, longbows are mainly used for pulling so while health below is a bad mod on one. also, if your health is below 50% you gonna die or get healed within a few seconds