Q. When someone bentches (recites Birchat Hamazon), before the Shekiah on Shabbos Chazon that is followed by Tisha Beav as it is this year (2021 – see question above), can he still eat after bentching until sunset, if he didn’t had in mind to do so and didn’t make a condition as required on the early morning before the beginning of a fast day?
A. Shaar Hatzion (553: 7) rules that there is no need to make a condition before bentching on that case., since one should not exhibit Inuy or affliction during Shabbos, most likely when he bentched he did not have in mind to stop eating until the Shekiah if he so desires. Kaf Hachaim (ibid. 6), Nitei Gavriel (91: 10) and other rule similarly.
Horav Shlomo Miller’s Shlit’a opinion is also similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a