Reference for Shabbat 300:1

ושל מה בכך מותר לחושבן

or of past expenditure may be calculated. But the following contradicts it: One may reckon up accounts that are not required, but one may not reckon up on the Sabbath accounts that are necessary. E.g., a man may say to his neighbour, 'I hired so many labourers for this field,' 'I expended so many <i>denarii</i> for this residence.' But he must not say to him, 'I have expended so much and am [yet] to expend so much'! — Then according to your reasoning, that [Baraitha] itself presents a difficulty.<span class="x" onmousemove="('comment',' The first Baraitha states in its first clause that one must not calculate past accounts, while the second clause states that past expenditure accounts are permitted. ');">¹</span>

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