Like the title says. To be more specific, a good example would be the fourth chapter/2nd extra story of "Akuma ga Hallelujah".

For the guy, he was just doing the love potion experiment for the money, and the girl used the potion against him for sweet revenge because of his cold attitude towards her. But, in the end, they fell in love unintentionally.

Something else that is sort of close to that summary would be perfect!