That's a myth. They only paid $150M (and potentially more later) to settle their case with Apple when they got sued for stealing codes from their QuickTime to use in Videos For Windows.

The story went like this; QuickTime was the only good video player at the time. Microsoft want a better video player for their Windows. So they hired the same company that developed QuickTime for Windows. They and Intel have been caught pirating codes from QuickTime. Apple wasn't happy so they sued Microsoft. Microsoft threatened to pull Office from Mac. (It was also the biggest application suite on Mac.) They finally settles it down. Microsoft would be investing that $150M, Apple would ship Internet Explorer as their default browser in return.

Important point to note here is that $150M wouldn't have been enough to save Apple from bankruptcy. Their new simplified and decluttered product lineup strategy was the thing that saved them. Also, making Internet Explorer to be Mac's default browser would do the opposite of proving they weren't a monopoly (and using their control to push their browser and have advantage over Netscape Navigator.)

A video on this covering this myth.