Excerpted from the great book Zen Antics, translated by Thomas Cleary (also in Classics of Buddhism and Zen, Volume Four).

Once when Zen master Seisetsu was seeing to the rebuilding of part of the monastery where he was teaching, a certain wealthy merchant came with a hundred ounces of gold, saying he wanted to donate it for the reconstruction project. Seisetsu took it without a word.

The next day the merchant came back to visit the Zen master. He remarked, “Although what I gave you was not so great an amount, it was an exceedingly costly donation for me. In spite of that, you didn’t say a word of thanks. Why is that?”

Seisetsu hollered, “I am planting your field of blessings; why should I thank you?”

The merchant was very embarrassed. He apologized and thanked the Zen master.