I'm kind of confused with this guy's effect, had an opponent discard this guy with Dealings, summon to my side, and then he discarded a card, because I was in control of it and it says your opponent must discard one card was that a legal move?

he's correct
its along the same as dark deal, where a normal spell card's effect is changed to (your opponent discards 1 card from their hand). It means, that you control the card, and now, as the controller, the controller's opponent has to discard

if he had use a card like gateway to the dark world to revive it, it would mean he is the controller, and the controller's opponent has to discard a card now