Another possibility is that Naruto and Yang have a special bond. This bond could be formed through shared experiences, like they both went through a difficult training together or survived a life - threatening situation. Because of this special connection, Naruto feels indebted to Yang or simply values Yang's life more than his own arm.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
One possible reason could be a sign of extreme loyalty and friendship. Naruto might see Yang in a dire situation where she needs his arm for some crucial purpose, like to complete a powerful jutsu or to defend against an overwhelming enemy. He is known for his selfless nature, so sacrificing his arm would be in line with his character.
It could add a lot of drama. The sacrifice of a limb is a huge deal, and it would change Naruto's abilities and his role in the story. It might make him more of an underdog, which could lead to new character development as he has to adapt to his new physical state.
Sasuke could be a candidate. Given their complex relationship, Sasuke might find a way to give Naruto a new arm as a way to atone for their past differences. He could have discovered some rare medical ninjutsu during his travels or come across a powerful artifact that he uses to create a new arm for Naruto.
It could be related to a mission. Suppose Naruto's arm has some special power or ability that is crucial for a mission, but Sakura disagrees with how Naruto is using or handling it, thus causing the confrontation.
Well, it could be that he got it as a gift from some super - advanced ninja technology group in the fanfiction world. Maybe they saw his need for an extra weapon or tool during battles and decided to equip him with a metal arm.
Maybe Orochimaru. Despite being a villain in the past, he is a master of forbidden jutsus and experiments. He could see Naruto as an interesting subject and give him a new arm with some dark powers, which Naruto would then have to learn to control for the greater good.
Well, in some 'naruto loses arm fanfiction', it might be a result of a forbidden jutsu gone wrong. Naruto might be exploring a new and dangerous form of power, and the price is his arm. This would then set off a chain of events where he has to find a way to either get his arm back or adapt. He could also face discrimination from some in the ninja community who see him as weakened, but he'd prove them wrong by his unwavering spirit.
One possible scenario could be Naruto having to relearn his ninja skills with only one arm. He might struggle at first but then find new ways to perform jutsus. For example, he could modify his Rasengan technique to be more one - handed friendly.
One special ability could be enhanced strength. The metal arm might be able to deliver much more powerful punches than his normal arm, allowing him to break through tough defenses more easily.