An excerpt from the commentary of Surah Yasin, by El Sheikh Syed Mubarak Ali Gilani about the sanctity of keeping a promise and placing Allah as the Witness and Fulfiller of that promise:
Trust is very important. When you make a promise and make Allah a witness, Allah will help you to fulfill it. If you make Allah a witness and you do not fulfill the promise, Allah’s curse will fall on you, because your words are law…. Do not dishonor your word.
A story is related of someone who needed 1000 dinars. He went to a money lender (of course not like the Jew) and he told the man, “I will give you back the money next year on such and such a date.”
The money lender said, “Bring some witnesses [according to Qur’anic injunctions].”
The man said, “My Allah is my witness.”
He said, “That’s fine, Allah is sufficient,” and lent the man the 1000 dinars. That man went away on a ship for trade. When he wanted to go back home, he could not find any ship returning there. Eventually, he took a log of wood, put the money inside, then put the piece of wood in the sea and said, “Oh Allah, I made you the witness. I entrust this money to the sea under your care. [Then he] commanded the sea, “Oh sea, I have borrowed this money on a trust [using Allah as a witness], take it to such and such a person.”
That man who lent the money came to the seashore one day, waiting for the ship to come. On exactly that day, he saw a log ashore and said, “Okay, the ship is not here, let me pick up the log and make a fire.”
He chopped up the log and found the money.