Some greenbacks, please.

请换些美钞。

Is there a chance that greenbacks are disappearing?

有机会让美元消失吗？

There were some loose bills in his vest pocket — greenbacks.

在他的西装背心口袋里有些零碎票子——绿颜色的美钞。

Some people are green with envy because a friend has more dollars or greenbacks.

有些人眼红朋友美元多，或者说是美钞多。

Dollars are called greenbacks because that is the color of the back side of the paper money.

美元之所以又叫“绿背”（美钞），是因为美钞的背面是绿色的。