The Divrei Chaim of Sanz was once conducting a siyum to celebrate the learning of all of Shas when he was stuck on the meaning of a specific Tosfos. He travelled to the author of Be’er Moshe, the rebbe of Kozhnitz and asked him for his help in understanding the difficult Tosfos.
Due to Rav Moshe’s great humility, he claimed that he did not understand the Tosfos and said, “You have already concluded studying all of Shas whereas I have just concluded saying all of Sefer Tehillim.”
The Sanzer Rav was undeterred. “Maybe you would agree then to an exchange? Let’s do a trade, you and I - I will trade you my study and siyum of all Shas for your siyum Sefer Tehillim?”
“No,” the Be’er Moshe said, shaking his head, “I won’t agree to trade.” Eventually he agreed to help the Divrei Chaim and they began to study the Tosfos together. When they were half way through, thanks to the Kozhnitzer, the Sanzer Rav understood! (Otzar Yisroel)
