Did not understand that line ... Implies NeXT had to pay Apple 400M?? In what sense?

Apple paid 400M to acquire NeXT, but NeXT's management ended up taking over the company. So really, NeXT paid -$400M to acquire Apple. It's like how Pixar acquired Disney for -$7.4B.

Software too. Mac OS X (and later, iOS) is a direct continuation of NeXT's NeXTSTEP and OPENSTEP OSes. Objective-C came to Apple from NeXT. All the Cocoa APIs used on the Mac come from NeXT, and the UIKit APIs used for iOS development are a direct descendant of them. The modern Apple contains very little of pre-1996 Apple but lots of pre-1996 NeXT.

It's a common joke that Apple paid for the privilege of being acquired by Next