The principle
What is offered for free is dangerous — usually a trick, or hidden obligations. What has worth is worth paying for. Generosity is a kind of leverage; suspect it.
The application
A favour accepted is a debt entered. Paying full price for something is the cleanest position there is, and it protects future negotiations more than any contract.
The defensive tell — how to spot it being run on you
When a vendor, partner, or peer keeps offering you generous unsolicited help, count the implicit owed. Sometimes the offer is honest. Often it is an opening move in a longer transaction you have not been told about.