I have this one since I've read SAFEKEEPER by @agnesclementineblog (i'm.literally obsessed with their tojishiu fics)
After Mamagumi's death, Toji couldn't gather the strength to get rid of her stuff for good, so he either put all the important stuff (wedding rings, photo album, gifts and souvenirs) in a cardbox and gave it to Shiu, or he kept everything the same at his place for a while until Shiu took things in hands and sorted out her stuff. He kept the things he thought Toji would ask for later, but Toji never mention it ever again. Shiu doesn't throw them out either. He keeps them safe for years and years in a safe.
In an universe where Toji lives and raises Megumi instead of selling him to the Zenin, one day, Megumi will ask about his mother. Or maybe say something about her that is so blatantly false that Toji gets mad at him.
"Don't talk about her like that, you don't know her."
"Obviously I don't since you never talk about her at all !! How am I supposed to know what she's like??"
And Toji realizes that he has to face his past for the good of his son, so he brings Megumi to Shiu's place and asks him for the box. It's been fifteen years but he doesn't even wonder if Shiu still has it, and it doesn't matter Shiu actually does. He brings the box to Megumi, shows him the rings and the old pictures, the keychain Toji bought for her birthday, a notebook where she used to note any idea that crossed her mind, from place she wanted to visit to jokes she found funny and even name ideas before Megumi's birth. And Toji finally talks, tells his son what it felt like to be married to that woman, how much she loved Megumi and how happy they were to have him in their lives. He sobs, hasn't done on a long time, so he feels stupid but Megumi cries too. Shiu quietly leaves the room to give them space. He returns an hour later when Toji calls him name. Megumi asks if he can have the box, and Shiu nods. It aches his heart because he had it for so long, it feels like getting rid of a part of himself. But this isn't his to begin with, he was only keeping it safe. It belongs to Toji and Megumi, it's only fair that they have it back. Shiu smiles and let them go, but right before getting out of the luxurious apartment, Toji turns to him, steals a kiss from his lips and mouthed silently "Thank you."
Shiu stands a few minutes in front of the closed door in his now empty apartment. Then he sighs softly, and goes on his balcony to smoke. As he reaches for his pocket for a lighter, his fingers meet a round object he's pretty sure he didn't have before. He takes it out and swears quietly.
"Fucking bastard," he smiled around his unlighted cigarette and rolls the ring on his palm.
Megumi gets home with a box full of things his mom cherished. Toji keeps living with the memory of the woman who taught him love. Shiu gets to keep Toji's wedding band from his marriage with his late wife. And he doesn't mind safekeeping for a bit longer. He sure hopes Toji would never ask it back.