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The Gemora relates the following: Mar bar Ameimar said to Rav Ashi: My father does business this way. If the sellers would come to him and say that they did not make this anticipated profit, he would believe them (without imposing an oath upon them; in such a case, he would only receive half of the actual profit, and not the amount written in the document; this way, he would avoid any ribbis concern).
Rav Ashi said: This is fine because your father is alive (and he will waive the amount written in the document). However, if he would die and the orphans would have a document saying that the merchants owed them a certain amount of money, wouldn’t they have to pay? This statement (talking about the death of Ameimar) was like an “error proceeding from a ruler,” and Ameimar died.
The Gemora in Moed Katan (18a) records the following incident: The Sage Shmuel paid a condolence visit to his brother Pinchas, who had lost a child. He asked him why he allowed his fingernails to grow although it was permitted to cut them. The rebuttal of Pinchas, “If such a tragedy as mine had befallen you, would you also show such disregard for mourning?” is described by the Gemora as an example of “an error proceeding forth from the ruler” (Koheles 10:5). The result of this apparent slip of the tongue was that Shmuel himself soon became a mourner because “there is a covenant for the lips” — a spoken word has the power to effect fulfillment. As proof of this power Rabbi Yochanan cites the statement made by the Patriarch Avraham, on his way to offer his son Yitzchak as a sacrifice, to the two young men accompanying them. “Stay here,” he told them “and I and the lad will return to you” (Bereishet 22:5), and did indeed result in their both returning.
How can we derive from here that a person should be careful with his words; this brought about a positive result that Yitzchak was saved?
The Chasam Sofer answers that to Avraham, it was regarded as undesirable, since he was unable to fulfill the will of Hashem.