Ăn mận trả đào.

Eat a plum, repay a peach.

The meaning is repaying kindness with a better gift as in “Ăn mận trả đào, ăn quả trả vàng / Eat a plum repay a peach, eat a fruit repay with gold” (i.e., peach is more valuable than plum, gold more valuable than fruit). This is similar to “Do unto others as you would have them do unto you.”