Perhaps the next Quarterly Panel Discussion.
A recurring theme in Heinlein from Between Planets 1951. where it was first articulated in that form. In The Moon is a Harsh Mistress 1966 the theme recurs explicitly several times.
As you read Heinlein you notice that Pay It Forward is really the moral philosophy that underlies all his writing.
A slight rework of a Religious moral philosophy "Do unto others, as they have done unto you" provides the same flavor. As we go through life, those in our society beginning of course with parents as the only society for an infant but expanding through the extended family, teachers and mentors in school, to those in the larger society do things for us, some of which we pay for but many of which we there is no way to compensate the source of the benefit.
Heinlein as an example has provided the basis for my personal morality, which seems to come down to a Pay it Forward imperative, and obviously I can't pay him back so all I can do is pay it forward, by helping others discover this alternative moral philosophy. As an interesting sidelight I am also paying forward a class I took at Stanford from Prof. Don Davidson as an undergraduate entitled Philosophy in Literature. He forever changed the way I read fiction. I still read the first time for the good story, but in many cases I will spend a much longer time on a second reading figuring out the philosophy that informs the story.