投我以桃，报之以李 He gave me a peach, I repaid him with a plum. (Book of Songs) Today shortened to 投桃报李, the friendly exchange of gifts and favors.
无德不报 No kindness goes unrepaid. (Book of Songs, the same poem)
结草衔环 To tie grass and carry rings in the beak. To repay kindness even after death. In the Zuo Zhuan, Wei Ke married off his late father’s concubine instead of burying her with him. In battle, her dead father tied the grass so that Wei Ke’s enemy tripped and was captured.