sufficiently wealthy people are morally obligated to donate some of their wealth to charity, for instance.

Yes, under utilitarianism, but I've already pointed out that any such imperative would be vastly exceeded by other imperatives.

I'm also not sure whether it works under deontological systems. One way to universalize such a claim is that you should pay people as much as they could earn under any state of affairs, but naturally that leads to you never making money and going broke as it's equivalent to demanding you give away all gains from trade: 'John makes me $1000 and currently costs me $500 in salary; however if the job market were much tighter, then in that counterfactual universe I'd be willing to pay him anywhere up to $999 but not higher, so I must morally pay him $999 right now - anything else would be exploitation and discrimination'.