Yeah same what do we do the with the cash as only coins work in the machines !

I think the way it works is 1 coin = 5 cash and vice versa. If you are not holding a weapon, you can see how much cash you have on the back of your left hand. When you pick up a wad of bills, it only adds 1 to the counter, but adds 5 when you pick up a coin. When you are near a vending machine and pull out a coin, you will see it drop by 5. If you have less than 5 total, you will not be able to pull out a coin until you collect more cash to equal at least 5. I don't think you'd ever be able to spend cash if you were down to your last 4.

It's a weird mechanic because it implies that some kind of magic conversion from cash to coin is happening in your backpack. Why have cash and coins instead of just coins? I guess only to make it feel like you are looting more by splitting them?

comon guys its alyx mechanics but with other visuals.. what did you do when you wehere upgrading for your gun in alyx ? you looked out for some more resisn sou you could complete 5 :d