Case:

Ruling: A worker may quit his job in the middle without losing the accrued salary. However, if the employer has to hire a new worker for more money to avoid loss, he can withhold that extra amount over the first worker’s projected wages (Shulchan Aruch, Choshen Mishpat 333:5-6). Loss of income which would have been attained had the worker continued working justifies doing so (Chazon Ish, Bava Kama 23:22). This applies to our case where it was justified for def to hire a new worker for a higher wage to try to rent the home on time. Def though cannot deduct for the loss of the rental. Although pl denies committing to work for a month, def is believed to say that he did because if def was willing to lie, he could have said that he already paid pl (migo).
The Ramban (Bava Metzia 76b) explains that the reason an employer can deduct the new worker’s salary is that the first worker is assumed to accept such losses when he takes the job. Based on this, the Pitchei Choshen reasons that if the worker was unaware that quitting the job he accepted could cause losses, he is not liable for such measures. In our case, pl was unaware of that situation. Similarly, the Nachalat Tzvi (CM 291:24) says that the rules of deducting from the worker to pay for a new worker apply only when he was hired for something that was known to include a loss. In contrast, if he was hired for a multitude of tasks that happen to include some that include losses when one stops in the middle, he can quit without having money withheld.
Even according to Tosafot (Bava Metzia 76b) that the withholding of pay in this case is due to dina d’garmi (payment for semi-direct damages), there is reason to exempt def in this case. That is because: 1) The Rambam (Sanhedrin 6:1) says that one is obligated to pay for garmi only when he damaged on purpose; 2) Since def knew that there would be losses, he should have informed pl. We cannot make pl lose for doing something that was not within his control to know.
Regarding calculation of his pay, the majority opinion was that since pl received more than the going rate, it is logical that this was on the understanding that he would in fact carry out the full project for which he was hired. Therefore, he will be paid for the work he did only at the rate of 20 shekels per hour.