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.