> What's the name of this law

It's a consequence of progressive tax rates.

Last year in the United States, for example, no matter how large your salary was, a single person's income tax rate for the first $10,000 of salary you earned was 10%.

The portion of your salary above that $10,000 is taxed at a higher rate, and as you cross salary amount thresholds (called tax brackets) the tax rate keeps getting increasing for the amount above that threshold.

By the time you get up above the $540,000, the portion of your salary above that $540,000 is taxed at the maximum 37% rate.

So if you purchase a big ticket item that is deducted from your salary before they figure out how much taxes you owe, high earners can save quite a bit.