i dunno what you want, but maybe this...: make a spell based on an ability that deals damage, and leaves a buff on the target. now make a trigger:
event:
unit starts the effect of an ability
condition:
abilty being cast equal to Heal (or i don't know what)
spell target unit has BUFF equal to true
action:
set HealDone = (the ammount that ability heals)
set spelltargetunit unit life to (life of spelltargetunit + HealDone/2)