吃人家的嘴軟，拿人家的手軟

Etymology
Literally, “the mouth that feeds from others is soft; the hand that takes from others is soft [as well]”.

In other words: “When eating dinner given by others, one should have a soft mouth; when taking bribes, one should have a soft hand.”

Proverb

 * 1) One may be bound to others because of the things they give you.