Reference for Bava Batra 173:1
ולגורן יפה סלע אסור להנות הימנו אבל אם שכרו מהיום בדינר ליום ולגורן יפה סלע מותר
but at that season [the labourer] was worth a <i>sela'</i><span class="x" onmousemove="('comment',' Sela' = four denarii. ');"><sup>1</sup></span> [a day] he must not derive any benefit from it.<span class="x" onmousemove="('comment',' Viz., the difference between the sum given and the price of labour at harvesting time; because this may be regarded as usury. since the labourer is paying a sela' (in labour) for every denarius he has received in advance. ');"><sup>2</sup></span> If, however, [a man] hires [a labourer to commence work] at once [and to continue through the harvesting season] for a denarius a day, [although] at the harvesting season he was worth a <i>sela'</i>, [he] is permitted [to pay in advance and to have the benefit of the difference].<span class="x" onmousemove="('comment',' Because the whole period of the hiring is considered as one long day; and, since on the first days of the period the labour was only worth a denarius per day. no higher price need be paid for the other days. ');"><sup>3</sup></span>