Or HaChaim on Deuteronomy 15:18/אור החיים על דברים ט״ו:י״ח

1 כי משנה שכר שכיר, "for twice as much as a hired hand, etc." Our initial reaction to these words would be that if the servant was sick for only three out of the six years he would not have to make up any time. Kidushin 15 derives from this verse that whereas a hired hand works only by day, a servant, i.e. עבד, works both by day and by night. Our verse alludes to this ruling.