Excess water after Netillath Yadayim

Question

Is excess water after Netillath Yadayim usable for other purposes? For example, you wash for bread, and you leave water in the cup - can someone use that water, or do they need to empty and refill the cup? Another example, you wash for bread with a normal drinking glass instead of a designated washing cup - a little bit later, but soon enough that there's still some water left in the cup, you want a drink of water. Can you use the cup without pouring out whatever is left at the bottom, or should you pour it out (or even wash it out) before using it?

Taking a more serious case (at least as I understand the Shu"'A), what about after washing in the morning upon waking up?

Answer 1

It has to be that you can reuse, for washing, the water left in the cup after washing, as otherwise you would have to refill the cup after each pouring on each hand.

Moreover, I found in memoirs of Rebbetzin Chana, the wife of Rabbi Levi Yitzchak Schneerson, that when he went into exile he did not have enough water to wash his hands. He would trade food with someone for water, would wash (morning) negel vasser, and the remaining water ("drops of water") he would drink. [This implies that water left after Netilas Yadayim is potable.]