Country: England
Even if they kept Mustafi salary the same, he was never signing anything for reasons you said yourself. Offering a reduced salary helped nobody in that scenario, I am fairly confident that didn't happen.What other reason could he have rejected it then?
It's probably hard for you to imagine anyone so sh*t ever wanting to actually leave Arsenal, but the reality for Mustafi is it was very uncomfortable there for him.
All besides the point anyways - this is about Saliba.