Return fires on the start of attacks though, and deals a set amount of damage. Try using a damage detection system, and whenever a unit takes damage and has the buff, cause it to deal X% of damage dealt to the attacking unit -- this will work for spells as well, if you don't want that, use an attack detection buff (envenomed spears) and add it to every unit, this will however cause buff placers to not stack with it, you can solve it by simply place buffs in the same trigger (if <Hero> has <Item or Ability> then create dummy and make it cast the according ability at the target).
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.