This is one of the arcs of a quick transmigration story, but I can't remember which one since I probably read it during the time where I binged on every QT story I could find. I'm not sure this arc was translated into English, but I'm 95% sure the series is on this site. Please help! orz MC transmigrates into the body of an orphaned kid (A), who was a talented pianist, in a world where musical skill is extremely valued. The kid was adopted by his deceased parents' friends, who had an older son (B), who was also his betrothed due to his deceased grandpa's wish. Same sex marriage was extremely common in this world, and A was sincerely in love with B, but B had no feelings for A. The only reason his adoptive parents managed to convince B to agree was because they already spent all of A's inheritance on their company, and wouldn't be able to return it if A left the family. But then, they decided to adopt another child (C) into the family, who was also very talented at piano. I don't remember if C truly fell for B or not, but the reason he started getting close to B was because he was jealous of A and wanted to break them apart. The three of them grew up together, but both B and his parents were very cold towards A, and grew more and more biased towards C, and A's talent towards music declined. With C's intentional interference, B fell for C and wanted to dissolve his engagement to A, but his parents wouldn't agree. B's parents also liked C more, because they also valued musical talent, and more importantly, the prestige it would bring to their family, but they couldn't let A leave their family with his no-longer-existing inheritance. If I recall correctly, A got into an accident (I don't think it was planned but I can't remember), and B took the chance to leave him for dead, so he could get A's inheritance and marry C at the same time. B is the absolute definition of scum. A had a beautiful white piano that his grandpa left him before his death, and it was A's treasure that he wouldn't let anyone touch. Seeing that, C asked B to sneak him into A's old house to play on that piano, and B actually took him there multiple times on their midnight dates. Once A died, all of his things belonged to B's family, and the piano was given to C. Of course, after MC transmigrated, his job was to change A's fate and face-slap the villains. C was no longer allowed to show off because MC got much better results at their music academy, B no longer fell in love with C, and their family got into trouble with the law because they couldn't return A's inheritance after the engagement was dissolved. IIRC, the ML in this arc was MC's music teacher.
Hi, I can’t remember which arc but it’s this story. https://www.novelupdates.com/series/cannon-fodder-cheat-system/