Shabbat 150B

ושל מה בכך ⁠— מותר לחשבן.

and of: What significance does it have, it is permitted to calculate them.

ורמינהו: חושבין חשבונות שאינן צריכין, ואין מחשבין חשבונות שצריכין, בשבת. כיצד? אומר אדם לחבירו: ״כך וכך פועלים הוצאתי על שדה זו, כך וכך דינרין הוצאתי על דירה זו״. אבל לא יאמר לו: ״כך וכך הוצאתי, וכך וכך אני עתיד להוציא״!

The Talmud raises a contradiction based on what was taught in another baraita: One may make calculations that are unnecessary, but one may not make calculations that are necessary on Shabbat. How so? One may say to another: I sent out such and such number of workers to this field, and I spent such and such number of dinar for this home. But he may not say to him: I spent such and such amount of money, and I am going to spend such and such amount in the future. Apparently, one is permitted to calculate one’s previous expenditures on Shabbat.

ולטעמיך, קשיא לך היא גופה! אלא: הא ⁠— דאיכא אגרא דאגירא גביה. הא ⁠— דליכא אגרא דאגירא גביה.

The Talmud responds: And according to your reasoning, it itself, the Tosefta quoted previously, is difficult for you, for it prohibits calculating past expenditures while allowing one to make calculations that do not have practical significance. Rather, it must be explained in the following manner: This Tosefta, which taught that it is prohibited to calculate past expenses, is referring to a case in which he has payment with him that he still owes his workers. Therefore, although his calculation pertains to projects that have already been completed, it is still relevant in a practical manner. And this baraita, which taught that it is permitted to calculate past expenses, is referring to a case in which he does not have payment with him that he must still pay his workers, and therefore his calculation does not have practical significance.