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modern-capitalism take that reduces to "People don't actually have principles, they only care about money"1
This is not a modern-capitalism take, maybe just a modern-capitalism phrasing. If you read "money" as common unit of value (which it is), not just dollars in hand specifically, then the statement is effectively true for 99%+ of people; the few for which it doesn't hold are considered insane[0], and we try to fix them or lock away in institutions, for the safety of everyone else[1].
There's no absolute value you won't compromise if the cost of keeping it is high enough, or equivalently, the benefit of breaking it is high enough. Having principles, integrity, is a way to boost the cost of some behaviors ahead of time, so you won't be tempted to engage in them even if presented with substantial rewards. It's particularly effective if other people know your stated principles and will hold you accountable. But it's still a finite threshold that can be overcome (especially with of companies, where principles generally are just pure marketing).
A skilled negotiator understands that everyone will sell out on their principles if the reward is big enough - but they also understand that it cuts both ways. Negotiation is a dance of allocating costs and benefits so that everyone actually gets something out of it[2]. It makes little sense otherwise.
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[0] - Absolute principles are like probabilities 0 and 1 in probability theory - they break the math. Or zero in general whenever you want to do multiplication or division. You don't want absolute principles, or anything you do will be defined purely by them.
[1] - And/or stuck in cults as followers; notably cult leaders tend to be very much pragmatic and money-oriented.
[2] - Relative to the non-negotiated/fought outcome. "Your money or your life" sucks for the mugging victim either way, but the victim prefers being alive to being dead and the mugger prefers theft to theft + murder, so a negotiation is still worthwhile for both.