I stole from my younger roommate. I stole her unused notebooks but I didn't steal high value items from her. I only get that because she was not using it and might be put to waste after. I don't know what to do. She was stealing back what's her mine from my bag and I didn't bother to talk to her about that matter.
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All stealing is a sin, high value or not. You took it without her permission. Perhaps she wasn't using the notebooks, yes, but you could've asked if she didn't need it anymore and if you could have it instead.
If it's in the original condition it was in when you first took it, give it back and apologise if you want to do the right thing. If it isn't in the original condition, still give it back and apologise. I'm sorry if I sound a bit harsh, but I'm just saying that this would be the right thing to do if you want Allah (SWT) to, instead of giving you sin, give you reward. Allah (SWT) is the most forgiving, but He is the most severe in punishment.