Pay it forward

[Justin Bieber]

This is a reference to the title phrase of the book by [Catherine Ryan Hyde].
In the book the phrase is used as the opposite of [payback]: when someone does a good dead for you, instead of paying them back, pay it forward by doing a good deed for someone else.

[Mark Zuckerberg]

A phrase used to imply or otherwise act upon the action in which one would perform a deed of good service onto, for, or in the favor of another person or group of persons with the sound intentions that the only thing they do in return is to perform a deed of good service onto, for, or in the favor of another person or group of persons where in which they should maintain the same sound intentions of the individual or group who performed a good deed unto them. This method of kindness is infallible unless met with a person of great greed and/or immaturity.

[Donald Trump]

Having sex with you wife in your sons bed because you know in 15 more years hes going to be doing the same to you.

[Vania]

to do something kind or useful for someone because someone else has done something kind or useful for you :