Say you work as a contractor that charges an hourly rate. If on any given day you feel sluggish due to poor sleep or some other reason, are you supposed to reduce your rate for that day to compensate for the amount of work you would have achieved if you would work 100%?
Shulchan Oruch Choshen Mishpot 337 (20) states the law (in the second part)
וכן חייב לעבוד בכל כחו שהרי יעקב הצדיק אמר "כי בכל כחי עבדתי את אביכן", לפיכך נטל שכרו בעוה"ז שנאמר "ויפרוץ האיש מאד מאד":
And similarly he (the worker) is obliged to work with all his strength because the righteous Yaakov said “for with all my strength I worked for your father” and therefore he took his reward in this world as it says “And the man became exceedingly wealthy”.
From this it would seem right to reduce your rate for that day to compensate for the amount of work you would have achieved if you would have worked with all your strength.
But Rabbi Yirmiyohu Kaganoff writes that:
In some areas of halacha, particularly the contract law rules for buying and hiring, there is a concept of minhag hamakom - that normative business practice determines what is halachically accepted. For this reason, the halacha regarding sales and employee rights are often governed by what is accepted normal practice. Since normal practice is heavily influenced by secular law, the halachic practice in these areas is influenced by the secular law. This is not because halacha recognizes the secular law but because accepted business practice is influenced by secular law.
From this it would appear that if normal practice would allow some leniency in the amount of strength expended, the worker may not be obliged to recompense his employer.
CYLOR in a practical case.