Ben was debating where to learn. Family circumstances required him to live at home, but the local options were not ideal. His first choice required driving quite a distance, which meant significant expenses.

Uncle Jack strongly encouraged Ben to learn where he would best succeed. “I am willing to cover two year’s worth of expenses,” he said. “When you fill up, save the receipts.”

“Are you serious?” asked Ben.

“Sure,” he replied. “I’ll even put it in writing.”

“Thanks! I appreciate it,” Ben said gratefully.

During the course of the year, though, events in the Persian Gulf caused gasoline prices to rise from $2.50 a gallon to $4.00.

One day, Uncle Jack joined Ben for the ride. On the way, Ben stopped to fill up with gasoline.

“Fill ’er up, please,” Ben told the attendant. “I’ll also need the receipt.”

The attendant filled the tank. “That comes to $80,” said the attendant. “Here’s your receipt.”

“Filling the tank costs so much now!” exclaimed Ben. “Much more than we expected!”

“What can I do?” said Uncle Jack. “That was the agreement I committed to!”

“But you probably expected prices to remain relatively stable,” argued Ben. “I’m uncomfortable asking you to pay so much. I’m not even sure that such an open-ended commitment is binding.”

“I don’t see why not; a commitment is a commitment,” said Uncle Jack. “But, if you want, we can check with Rabbi Dayan about it.”

The two went to Rabbi Dayan. “Uncle Jack committed to cover my gasoline expenses for two years,” began Ben. “Is such a commitment binding, especially with the steady price rise?”

“If the price rise is within reason,” ruled Rabbi Dayan, “according to almost all authorities, the obligation is binding.”

“Why should anyone disagree?” Ben asked.

“This case is one of an unlimited obligation, davar she’eino katzuv,” replied Rabbi Dayan. “The Rambam (Hil. Mechirah 11:16) is of the opinion that an unlimited obligation is not legally binding, other than in specific circumstances. Since the person does not know how much he is obligating himself to, it is similar to asmachta, an insincere commitment.

“Almost all other Rishonim disagree with the Rambam,” continued Rabbi Dayan. “They understand that a person can commit with a kinyan even to an unlimited obligation.

“What is the source of this dispute?”

“The Mishnah (Kesubos 101b) deals with a person who marries a woman with a daughter from a previous marriage and commits to support the daughter for a number of years,” explained Rabbi Dayan. “The man is obligated the support his stepdaughter whether prices rise or fall. The Gemara (102a) explains that because of the special circumstance of mutual commitment in marriage, this obligation is binding verbally; there is not even a need for a kinyan” (E.H. 114:1).

“Here, the amount of support is undefined, as the cost of living is variable,” continued Rabbi Dayan. “The Rambam apparently understood that such a commitment is generally not valid at all, even with a kinyan, whereas the other Rishonim understand that a kinyan is required in general, but the commitment is always valid.”

“What does the Shulchan Aruch rule?” asked Uncle Jack.

“The Shulchan Aruch (C.M. 60:2; 207:21) cites the Rambam, but writes that all subsequent authorities disagree with him and the halachah is like them,” replied Rabbi Dayan. “Despite this, some Acharonim apply kim li, ‘to follow’ the lenient opinion of the Rambam and not enforce an unlimited commitment (see Yabia Omer, C.M. 3:4). However, the Shach (60:12) concludes that since all subsequent authorities disagree with the Rambam, as the Shulchan Aruch writes, one cannot even apply kim li.

“Therefore,” concluded Rabbi Dayan, “Uncle Jack’s commitment is binding, even though prices rose significantly. However, if the cost would skyrocket unreasonably — e.g., more than double — some authorities maintain that he never committed to this, and would not be liable for that price.” (See Beis Shmuel E.H. 114:3; Avnei Miluim 114:2; Pischei Choshen, Kinyanim 18[27].)