A simple test shows that it is not possible to decrease a hero's experience directly. The native functions simply refuse to set a hero's experience to a new value if it is lower than the hero's current experience. The same is true for trying to set a hero's level, however the BJ function suggests there is a way:
JASS:
function SetHeroLevelBJ takes unit whichHero, integer newLevel, boolean showEyeCandy returns nothing
local integer oldLevel = GetHeroLevel(whichHero)
if (newLevel > oldLevel) then
call SetHeroLevel(whichHero, newLevel, showEyeCandy)
elseif (newLevel < oldLevel) then
call UnitStripHeroLevel(whichHero, oldLevel - newLevel)
else
endif
endfunction
The
UnitStripHeroLevel function lets you reduce a hero's level. You could use it to reduce the hero's level until his current experience is less than the target experience, then add the difference to reach the target.
The problem is that
UnitStripHeroLevel seems to have some unavoidable side effects. It will automatically unlearn abilities when the hero goes down a level. You can avoid this by adding a skill point to the hero before reducing his level, but in some cases an ability will still get unlearned so doing this is pointless. One case is if you already have all your abilities learned since in that case you can't add any more skill points to the hero. Another case is when the hero's level is reduced below the level skip requirements of an already learned ability.
There's even a bug here that can cause your hero to loose skill points in some ability level requirements configurations. For example, if your hero can learn multiple abilities at level 1 that each have a 3 level skip requirement (WC3's default is 2), then if you save up your skill points you can get two abilities to level 2 once your hero hits level 4. If the level is decreased back to 3, both abilities will have their level decreased to 1, but the hero won't get one skill point back like he should (since he only went down one level, but had two skills deleveled). This is luckily not a problem with the default WC3 hero skill setup where you can't save up skill points in this way.
So, the solution described earlier would need to save the ability levels of all hero skills on the hero (this means you'd need a database of all heroes and their skills). This is because the experience reduction we are aiming for will not necessarily take him down a level, so once we add the experience back to the hero to get the target reduction, the hero will gain the level back and get a skill point to spend. That'll look weird if you don't automatically spend it for him on the same ability that was lost when deleveling the hero, but to know which ability that was you need the database mentioned earlier.
