Someone who left his place without saying Birkat Hamazon doesn’t need to go back to where he ate, he is allowed to say it where he remembers. If he is able to return, he will receive extra bracha, as long his returning wont be after 72 minutes. If he still has Kazayit of bred still that he took iwht him then he should eat it and then say Birkat Hamazon.
Explanation:
The Mishna in brachot 51b: He who ate and forgot to say Birkat Hamazon, Bet Shammai said he must return to his place and say Birkat Hamazon, and Beit Hillel said he should say it where he remembered. Until when can he say it? Until the food is digested.
The Gemara (53b) comments: Rav Zvid an some say Rav Dimi bar Abba said the argument between B”S and B”H is even he forgot but if left on purpose everyone agrees he must go back. Of course, the Mishna says he forgot? YOU might say the Ishan means even if he did it on purpose and we said ‘forgot’ to shoe that B”S is stringent even then, but that is not correct. A Beraita said, B”H said to B”S according to you one who eats at the top of the building and forgot to say the bracha has to go back to the top? The answered according to you one who forgets his wallet at the top of the building he should not go up and retrieve it ? he definitely should and if he goes up for himself of course he should go up from Hashem.
The Rishonim disagree fi we rule like B”S or B”H. The Rosh and the Rabbeinu Yonah in the name of the Gaonim rule like B”S. The Rambam and the Rabbeinu Yonah rule like B”H.
One big difference in halacha between the two options is if one left accidentally whether you have to return or not. According the Rosh and B”S it seems the is no difference but the Tur writes if he actually di say Birkat Hamazon where he remembered if it was accidental then Bdiavad he is does not have to say it again. But if he did it on purpose, he just go back and say it again. Meaning there is a difference between accidental and on purpose Bdiavad.
So, there is a difference between the ruling of the rosha dn the Rambam. According to the Rosh, if you left and intended to say Birkat Hamazon and different pale then even if you said it you did not fulfill your obligation. According to the Rambam who rules like Bet Hillel, the Bet Yosef says you do not have to say it over in the correct place.
The Taz agues on the Bet Yosef and understands there is no argument between B”S and B”H everyone agrees if you said Birkat Hamazon in the second place accidentally you fulfilled your obligation and if you purposefully said Birkat Hamazon in the second place you are not.
This disagreement between the Taz and the Bet Yosef is actually in Bet Shammai himself. If he left accidentally even if you should go back if you didn’t go back on purpose according the Bet Yosef you fulfilled your obligation but according the Taz you don’t, since you did it on purpose.
The Shulchan Aruch (OH 184:1)writes: One who ate needs to say Birkat Hamazon before he leaves. If he left and didn’t say it. If he did it on purpose he must go back and say Birkat Hamazon there. If he said in the place where he remembers he fulfills his obligation. The Rema writes this is only according to the Rambam but the Rosh holds that even accidentally he must go back to his place and if did it purposefully then he does not fulfill his obligation. The Shulchan Aruch continues: if he did accidently according to the Rambam he says the bracha where he remembers according to the Rosh he must go back.
It seems the Shulchan Aruch that he follows the opinion of the Rambam if one purposefully if he said Birkat Hamazon he fulfills his obligation Bdiavad, unlike the Rema who rules like the Rosh. It would seem then in the end of the Shulchan Aruch he follows the Rambam that he does not have to go back to his place. The Mishna Berura though that if he leaves accidentally if he does go back then he would be praised.
The Yalkut Yosef agrees.
In this case whether you went on a bus, to return to the original place might be more than 72 minutes just say it on the bus.
