Robert Greene's Law 40 from The 48 Laws of Power: nothing in human dealings is ever truly free. When someone offers a service, gift, or favor with no price tag attached, they are almost always pursuing a hidden agenda — quietly accepting it creates a felt obligation that distracts the recipient from noticing what the giver actually wants in return. Greene allows one near-exception, a mother's love, and even that, he says, usually expects something back.
Applied specifically to money and employees: being cheap or stingy with pay is a "terrible, terrible quality" that signals weakness rather than discipline, and it erodes the relationship over time. Paying people well — while still keeping them a little on their toes — builds a stronger, more confident position than being generous with credit or praise but tight with money. The related idea is that durable teams and partnerships need each side to have skin in the game (paid, mutually invested commitments) rather than one party working for free.
Being given something for free (a book, a t-shirt, a "free" call) should trigger immediate wariness about what the giver wants in return — even though mass-market brands also use free giveaways as legitimate, low-stakes marketing. The tell is scale and context: a stranger's unsolicited free offer of real value is a signal to read, not a gift to simply accept.