During a Simchas Torah farbrengen, the Rebbe made a promise that pledges to tzedaka made at the farbrengen would bring a five-times return. When some people came forward after Yom Tov, the Rebbe said it was too late.
At the Simchas Torah farbrengen of 5740 (1979), the Rebbe suddenly grew serious and began, “A Yid once received a bracha from my father-in-law, that whatever he gives to tzedaka he will see returned to him multiplied by five. Indeed, this is what happened. Likewise, today, whoever gives tzedaka beyond his means will see it returned to him multiplied by five. He will be able to count it with his fingers, as a Litvak would!”
After these words, a niggun was started. The Rebbe sat for a while with closed eyes and a very solemn expression.
The Rebbe opened his eyes and said, “People will come after Simchas Torah and ask if I was serious. There is nothing to ask… But don’t come [to pledge] after Simchas Torah, when we’re overcome with measured limitations.”
Many of the assembled took upon themselves great sums for tzedaka. However, some only realized what was happening after yom tov and approached the mazkirus to give a donation.
At the next farbrengen the Rebbe said, “Whoever decided then, his donation will be accepted. But if not, he should take it back. If he won’t, I will have to send it back…”
(Sichos Kodesh 5740; Parshios Im Harebi, Bereishis, Page 235)
From The Weekly Farbrengen by Merkaz Anash