So far I've read the LN until vol 6 (will read the rest later). If Lapis really wanted him to be full of dept, the sword alone if enough, by witch I mean, iits value is so astronomical that he would never be able to pay it back even in several life times.
This sword is actually Lapis's father sword, witch she took(for some rebelious revenge sake) and made a deal with the shopkeeper to hold until she come to take it back (while giving the shopkeeper a few gold coin (the fake price they "bought" it)), and the other equipements included.
And honestly, so far I really like their dynamic and characters (along with Sena, yes ,she's a major character in story).
Going back to the last part of your comment, yes it seem that way for now, because he know that at his adv rank(bronze) , he won't get anything at all, and it will take him a long time to pay back. Should also add in the fact she actually doesn't really care about the money, she just to want a reliable/decent person as partner in her search of her parts (and he knows that) ,he fits the requirement and he doesn't have much to do as well. But that will be for the later.
It's somewhat frustrating that the main character is so sharp and skeptical around every single character in the series, but for some reason (plot convenience?) has a massive blind spot when it comes to the demon priest girl. It would have been easy for the character to simply turn her down and earn back the money he owes her on his own, but instead he's somehow drawn to a character with little to no redeeming personality to the point where he essentially lets her walk all over him.