Yeah, once they get to the story in vol 5 of the LN, you'll see why the mangaka put it that way.....
Real spoiler (don't get mad at me if you click and get spoiled):
After Sarasa saved the Lotze family from the Baron, they (the Lotzes) have announced that Kate will be married to Sasara (same sex marriage is legal, but rare amongst nobles - there's a high-ranking Marquess who's in a same sex marriage to another woman). Everybody is also speculating that Iris will be the "mistress".
So in other words, in order to clear their debt (which iirc in the anime is like 32m GP atm?), Kate gets sold to their debtor.
TBF i watched the anime because i thought it would be a chill dumpster fire watch and i had just watched I shall Survive Using Potions and some other alchemist anime i cant remember at all. so im not too concerned about spoilers. the problem is this is really the best of the 3 since Potions! is frankly bad compared against FUNA's other works once Kaoru leaves Balmore its not great until we get Reiko and Kyoko into the picture after the second timeskip and the story returns to Kaoru being an insane schemer, and the other clearly didnt leave enough impression on me to care to remember its name at all