I think he paid too much money to the duke.
I could see him getting a promotion is very beneficial for the duke. He isn't without ties to the dukedom - he has Angie by his side. However, he doesn't have any ties with anyone else as he is an upstart. And then he got some renown for being a successful adventurer and beating those 5 in duels. Add to that his noble rationale for fighting the prince. I think the duke sees a potential for him to be a huge asset for the dukedom, especially if him and Angie ever get married. But for any marriage to happen, MC has to be of a befitting noble rank.
If he does ever get promoted to a high enough rank, Angie will be "in his strike zone", and Olivia will too eventually when she becomes a saint.