“You reap what you sow.”
Another term that can be used for this is the effort put in is the reward received. It’s like a good form of karma.
“What goes around, comes around.”
A smile given is a smile received. My favorite version of this saying is an actual, true story.
My mother went into a drive through at Starbucks. She was single with me and my sister and was rather broke. A gentlemen in the car in front of her paid for her Starbucks, and we were all so thankful! The next time this happened, my mother paid for the person behind her and that person paid for the person behind her and the chain continued almost all day! Kindness given is kindness received (usually/eventually).
However, if one is in a position where they are constantly kind and receive harsh treatment or abuse, it is okay to leave such a situation. It is self-respect, not giving up on that person!