For us(I'm Arabic), we usually say it when someone comes and seems upset, or is about to share something and we suspect it to be bad news (example: awaiting a response from someone who recently endured hardships)
So the person looks at the other and says "Kheir InshAllah?" In a way of saying, " God willing, you come with good news?"
It's kind of like an expression. If it is not used in that manner, then "Kher inshAllah" can also be said after someone does something, or takes a decision of some kind. Then you say " Kher inshAllah" as though you are saying " God willing,good will come out of it." hope this helps.