Well, apparently the Mourish Occupation of the Iberian Peninsula was what brought to the Dark Ages Europe advanced irrigation techniques which spread from there increasing agricultural production, the growth of cities and ultimatelly the Renaissance, so we probably would need to pay them, or at least their descendants (mainly Northern African Arabs).
I see. This means that if any invader improved a property they took over in any sense of the word the takeover was justified. If for example I steal your car but give a good wash. You should thank me for cleaning it and I was right to steal it, since you were neglecting it.