On the Gemara which says: “Anyone who does work on Tisha B’Av won’t see any blessing forever (אינו רואה סימן ברכה לעולם)”, Tosfos explains: כלומר באותה מלאכה שרגיל לעשות בת''ב אינו רואה סימן ברכה לעולם - “Meaning to say, in the job that one does on Tisha B’Av, he won’t see any blessing forever”.
The Elyah Rabbah (554:26) quoting the Tanya explains, that from the money which one gained from the work done on Tisha B’Av, one won’t see any berachah. However, the Machatzis HaShekel (554:27) is medayak [makes an implication] from Tosfos, that if one does the melachah [job] that he did on Tisha B’Av during the year, he won’t see any berachah from that particular type of melachah. Once one did a particular melachah on Tisha B’Av, he will never see berachah from that particular melachah again.
The Machatzis HaShekel is saying a tremendous chiddush, however, we find precedent to such a chiddush from a Tosfos HaRosh in an entirely unrelated sugya in Megillah. The Gemara in Megillah (32a) says:כל האוחז ס״ת ערום נקבר ערום בלא מצוות – “Anyone who holds a sefer Torah without its cover, he will be buried naked (i.e. without) from mitzvos”. The Gemara explains: בלא אותה מצוה – “without that particular mitzvah of holding a sefer Torah”.
The Tosfos HaRosh explains: Even if one regularly holds a sefer Torah and rolls it, if he once held it without its cloth, he loses out on all the gelilas [rolling’s] that he has done throughout his life. Similarly, if one regularly reads from the sefer Torah, and once he touched it without using cloth, he loses out on all the reward for the readings he has done until now.”
The Machzik Berachah (siman 147) writes: According to this, the expression ערום בלא מצוות – “naked without mitzvos”, written in plural is understandable. As we are coming to say, if one touched the sefer Torah even just once without a cloth, he loses out on the reward for all the mitzvos he has done until now, not just the reward for that particular time.
