The difference between the rates, 20 points in this case, is known as the "spread" or "margin".

买进和卖出汇率的差额------以这里20点为例称为“差幅”。

youdao

First and foremost, ever-present low interest rates have squeezed bank profitability as measured by net interest margin, or the spread between what banks pay to borrow and what they're paid to lend.

WSJ: Standard & Poor's 500 Had a Good Year in 2011 -- If You Don't Count Too-Big-To-Fail Banks