It is not appropriate for us to determine whether the talmid chacham was right or wrong, all the more so because we do not know the exact case and all of his reasoning. However, we will gladly share our understanding of the matter of mechila in similar cases, which will shed light only on some of the possible scenarios.
The main issue here is mechila b’lev (mental relinquishment of rights). In contrast to most financial dealings, mechila does not require a kinyan (act of finalization).[1] Oral mechila, even in the absence of the person who is obligated, is binding, and one who was mochel can no longer demand payment.[2] The K’tzot HaChoshen[3] cites the Maharshal, who says that mechila can even be accomplished mentally. As proof, he cites a gemara[4] regarding a widow who did not request payment of her ketuba for twenty-five years after her husband’s death.The gemara rules she can no longer demand payment because of the assumption that she was mochelet. Since there is no indication that she verbalized this mechila, the Maharshal concludes that mechila b’lev is effective.
The K’tzot HaChoshen rejects the Maharshal’s opinion due to the general rule that “matters of the heart are not binding.”[5] Instead, he accepts the following distinction of the Maharit.[6] When there is a presumption throughout society of mechila (e.g., regarding the ketuba), it does not need to be verbalized. However, mechila that an individual may have contemplated in a situation in which others might not have is not effective without verbalization.
The rationale for this distinction is practical and logical.[7] In many areas of life, one regularly vacillates between options before arriving at a decision. He might have been “sure” at one point but then later decided upon the opposite. It is unfair to bind someone to a decision unless he was aware that after a given point, he would be unable to change his mind. Regarding most monetary matters, only a kinyan indicates and effectuates finality. When it comes to mechila, however, speech is deemed sufficient; we expect a person to weigh his resolve before verbalizing his mechila. However, thought, which does not necessarily lead to a decision, is not enough. Only if a person is in a situation in which all would arrive at the same final decision and he provides no contrary indication can we assume that he was fully mochel like everyone else.
In spite of this rationale, there are significant opinions on both sides of the question of whether mechila b’lev is effective in cases in which it is not obvious.[8] Our feeling is that the stronger position is that it is not binding. Note that even according to the Maharshal, the level of finality in one’s mental mechila must be compelling. If one thought to himself, “I’m too tired to go now, so I guess I will forgo the money,” it is meaningless. The Mahrashal is talking about a case in which one decided at some point that he will not demand the money. It is questionable whether the person who planned not to return milk because of the trouble involved had reached a sufficient level of finality.
On the other hand, an action that suggests mechila can be binding even if it is not performed explicitly for the purpose of expressing the mechila. A classic case is one in which the action would be improper if one were sticking to his rights. Applying this idea to our basic scenario, if the milk was barely edible and one put some in his coffee anyway, that would be an indication he was mochel his right to return the milk; otherwise, the milk would not be his to use.[9]
A final factor to consider is that even if mechila b’lev is generally binding, there is an exception in the case of mechila b’ta’ut (based on a false premise). In our scenario, for example, had the buyer known that his son was going to the store, he might not have been mochel. In general, processes that are done b’ta’ut, whether mechila or kinyan, are invalid.[10] However, this happens only when the unknown circumstance existed at the time of the mechila or kinyan. If the circumstance developed afterward, the process would be valid.[11] As an example in our scenario, if the son decided to go to the store only after his father was mochel, the mechila would not be considered to be b’ta’ut.
To sum up, in the story in which you cite the talmid chacham’s ruling, there are cases in which the buyer would have been mochel and therefore could not return the milk, but there are also cases in which his thoughts of not returning the milk would not constitute binding mechila.