Continuing on the theme of the nature of mitzvos derabbanan, the Ba’al HaMaor (daf 7 in R"IF to R”H) writes that someone who is mudar hana’ah from another is still permitted to blow shofar for him on Rosh haShana because mitzvos lav l’henos nitnu. However, he is not permitted to blow chatzotzros on ta’anis as this is only a mitzvah derabbanan. At first glance one would have thought the opposite should be true – if helping someone fulfill a mitzvah d’oraysa which is a serious responsibility is not considered hana’ah, certainly helping them fulfill a less stringent derabbanan should not be considered hana’ah?
R’ Shimon Shkop explains the Ba’al haMaor based on the idea suggested in this previous post. We apply the principle of mitzvos lav l’henos nitnu to mitzvos d’oraysa because the act is inherently defined as a ma’aseh mitzvah. However, a mitzvah derabbanan is not inherently a ma’aseh mitzvah; its performance is just a means to accomplish the goal of obeying Chazal and not violating "lo tasur". Therefore, since the act itself is not a mitvah, the neder is chal.
This is another opportunity to compare/contrast R’ Shimon and Brisk. The Mesorah Journal (vol 5) presents the R’ Solovetichik’s approach to the same Ba’al haMaor. Nedarim prohibit acts done for the sake of hana’ah. RYBS argues (see post here for more on the topic) that where the Torah labels an act as a mitzvah, it cannot be excluded by a neder because it is by definition not an act of hana’ah. However, by dinim derabbanan, the act itself is not defined by the Torah as a ma’aseh mitzvah and therefore the hana’ah received is considered a direct benefit resulting from the performance.
The difference between the approaches is subtle, but I think it is there. R’ Shimon likes to look at causes. What is the goreim, the sibah? Both dinim d’oraysa and derabbanan are “ma’asei mitzvah”, but one is intrinsically wrong and the other is just a means to avoid lo tasur. Brisk is not about underlying causes, it’s about labels. A derabbanan is “ain sh’ma ma’aseh mitzvah klal”, it cannot be labeled a mitzvah act. It’s not enough to say it is a ma’aseh mitzvah but has a different goreim because philosophizing about reasons and causes has no place in the world of Brisk. There is little practical difference between the explanations here, but I think it does reveal different modes of thinking.
On a final note: R’ Shimon does not address the problem, but see the Mesorah Journal article’s explanation on why the Ba’al haMaor calls only tekiyos on R”H d’orasya when themitzvah of chatzotzros is also derived from a pasuk.