A man could not afford the wine for the four cups of the seder, and asked the Rav whether he could use milk. The Rav offered to pay for wine, and gave him much more money than wine would require. The wife of the Rav overheard the transaction and later asked the Rav why he had provided so much money. The Rav replied that because the petitioner had suggested milk, it is obvious that he cannot afford meat either.
What's the source of this story? Did it actually occur?