לזכות אבי מורי ואמי מורתי שיחיו
Tosfos in Yevamos [ה: ד"ה כולכם] asked why we need a pasuk to teach that כיבוד אב ואם is not דוחה a לא תעשה. Instead, we should derive this from the fact that both you and your father are commanded to keep Shabbos, just as we find [Kiddushin 31] that one honors his father before his mother because both he and his mother are obligated in the honor of the father. Answered Tosfos that we learn there from here. The source that one honors one's father before his mother because both he and his mother are obligated in the honor of the father is the sugya in Yevamos which teaches that כיבוד אב ואם is not דוחה a לא תעשה because both the son and the father must honor Hashem and the Shabbos.
Tosfos in Yevamos [ה: ד"ה כולכם] asked why we need a pasuk to teach that כיבוד אב ואם is not דוחה a לא תעשה. Instead, we should derive this from the fact that both you and your father are commanded to keep Shabbos, just as we find [Kiddushin 31] that one honors his father before his mother because both he and his mother are obligated in the honor of the father. Answered Tosfos that we learn there from here. The source that one honors one's father before his mother because both he and his mother are obligated in the honor of the father is the sugya in Yevamos which teaches that כיבוד אב ואם is not דוחה a לא תעשה because both the son and the father must honor Hashem and the Shabbos.
This is difficult to digest! The Gemara wanted to learn כיבוד אב ואם as a prototype for the whole Torah that an עשה should be דוחה a לא תעשה. Then the Gemara rejected this only on the grounds that כיבוד אב ואם is a הכשר מצוה [see Rashi and Tosfos], implying that otherwise, כיבוד אב would be דוחה. But how can this be? אתה ואביך חייבים בכבודו - Both you and your father are obligated in the honor of Hashem?? So how can we say that all things being equal כיבוד אב would be דוחה a לא תעשה?! [See Reb Elchonon - the real one, not the fake one writing this post - קובץ הערות סימן י"ח].
The Ramban and Rashba [Bava Metzia 30a] explained the concept of "לא דחינן איסורא קמי ממונא" a case of איסור is not overridden by monetary considerations as being akin to the Gemara in Ksubos [40a] that there is no עשה דוחה לא תעשה in the עשה of ולו תהיה לאשה [being forced to marry and support the victim of rape], "כיון דאי אמרה לא בעינא ליתא לעשה כלל" - since if she says that she is not interested there is no עשה. Meaning that an איסור can't be overridden because the other person has a mitzva, for both are obligated in the honor of Hashem. So in a case where the rapist is halachically forbidden to her, she should say that she is not interested in marrying him thereby avoiding the forbidden relationship. In the same way, one should say about monetary benefit that he could accrue only through איסור that he is not interested in the money and thereby avoid having to override an איסור. The mitzva of כיבוד אב is an example of a monetary obligation of the child to provide for the parents that the parents may forgo if they so desire. Hence, it doesn't override איסור.
Here is the language of the Ramban:
ונראה לי דהכי פירושה משום דכיון דאפילו בעל אבדה כהן אינו רשאי להטמא בה היאך יטמא זה בשבילה, והרי אם באו בעלים ואמרו לא בעי' לה ליתיה לעשה כלל, הילכך לא דחינן מצות עשה משום מצוה שבממון שאם הפקיר בעליו ממון זה פטור הוא מאותה מצוה, שהרי חייב הוא לומר כן כדי שלא יטמא כהן ששניהן חייבין בכבודו של מקום והתורה אמרה להחזיר אבדה לחברו ולכבד אביו ואמו בממונו ולא לעבור על המצות, ודמיא לההיא דגרסינן בכתובות (מ' א') היכא אמרי' דאתי עשה ודחי את ל"ת כגון מילה בצרעת דלא אפשר לקיומיה לעשה אבל הכא אי אמרה לא בעינא ליה מי איתיה לעשה כלל, כלומר הואיל ואפשר לה לבטלו אף על פי שאמרה בעינא ליה אין שומעין לה ששניהם חייבין בכבודו של מקום ולא גלי רחמנא בכי האי עשה דנדחי, וכל שכן הוא בדבר שבממון דכל שבשלו אינו רשאי בשל חברו נמי אינו רשאי.
The idea that כיבוד אב doesn't override איסור because both parties are obligated in the honor of Hashem is exactly what the Tosfos we saw was saying. But that once again will bring us into a quagmire because the Ramban and Rashba explained the Gemara in Yevamos as saying that since the act of cooking for a parent is not an inherent mitzva but a הכשר מצוה, it does not override Shabbos. That would imply that otherwise it WOULD override Shabbos. But how can that be in light of the fact that both the parent and child are obligated to honor Hashem?!
Indeed, Tosfos Yeshanim [Ksubos 40a] asked why we need a pasuk to teach us that כיבוד אב doesn't override a לא תעשה. Why don't we just say that כיבוד אב doesn't override איסור because the parent can say that he doesn't want the service and thus avoid the איסור!? So the simple answer is that the very דין of being able to say "לא בעינא" is rooted in the sugya that proves from a pasuk that כיבוד אב isn't דוחה איסור. So it is only AFTER that sugya that we know the סברא that if one can say "לא בעינא", there is no דחיית איסור. But as we saw - from that very sugya we see that this is not true [because only the factor of הכשר מצוה determines that there is no דחיית איסור]. So the reason MUST be that "לא בעינא" is a סברא with no source in the psukim, which illuminates the question of the Tosfos Yeshanim - if we have such a סברא, why do we need a pasuk to teach that כיבוד אב isn't דוחה an עשה?!
