Joe and Arvid, in my opinion, both of you seem a little confused on how Seduction and Manipulation work in the game.
Both of them are trying to get someone to do something you want via some kind of leverage. It's seduction when the leverage is sex.
The leverage doesn't necessarily have anything to do with the promise.
So you threaten a dude with a knife, or offer the dude sex, in order to get him to, I dunno, try to get him to back you with his gang when you try to take over the holding. As it says in the book, on a hit, they ask you for a promise. The thing they want in return doesn't necessarily (and probably usually shouldn't, in my opinion) have anything to do with the leverage. So this dude, he says, if I help you take over the holding, I want all of your next crop of deathweed.
The (probably implied) offer of sex, if you're seducing, or the threatening with the knife... that was just the shit you used to get him to listen to you in the first place.
That's how I see seduction and manipulation, anyway. I may very well be totally wrong.