金剛經纂要刊定記 2.9

2.9

酬因者，酬謂酬報，因謂因地。以佛於因地初發心時，希求無上正等菩提，遂啟四弘誓願：煩惱無邊誓願斷，法門無邊誓願學，眾生無邊誓願度，佛道無上誓願成。於此四中，三願皆畢唯一未圓，誓度眾生眾生宛在。今雖證果不捨因門，現身說法濟度群品，以報先願，故曰酬因。故《法華》云：「我本立誓願，欲令一切眾，如我等無異。如我昔所願，今者以滿足，化一切眾生，皆令入佛道。」

As for "repaying the cause": "repay" means repaying, and "cause" means the causal stage. Because when the Buddha first generated the mind at the causal stage, he sought unexcelled complete bodhi and thereupon initiated the four great vows: "Afflictions are boundless, I vow to cut them off; Dharma gates are boundless, I vow to study them; sentient beings are boundless, I vow to deliver them; the Buddha Way is unexcelled, I vow to accomplish it." Among these four, three vows were all completed and only one was not yet fulfilled: the vow to deliver sentient beings, while sentient beings are still clearly present. Now, although he has realized the fruition, he does not abandon the causal approach; he manifests a body to expound the Dharma to rescue the multitude of beings so as to repay his former vow, hence it is called "repaying the cause." Thus the Lotus Sūtra says: "I originally made a vow, wishing to make all beings equal to me without difference. As I formerly vowed, it is now fulfilled: transforming all sentient beings and causing them all to enter the Buddha Way."