Gemara

75b (Mishnah): If Reuven hired a donkey-driver to bring flutes for a wedding or funeral, or workers to take soaking flax out of the water, or any Davar ha'Avud (he will lose if the job is not completed now), and they retracted, if there are no other workers there (to do the job for the contracted wage), he hires others for more (and those who retracted must pay the excess), or tricks the retractors.

76b (Beraisa): If it is Davar ha'Avud, Reuven hires others for more, or he tricks Shimon (the worker). To trick him, he says 'originally I promised one Sela. Now I offer you two' (but he will pay only one). One may pay other workers an extra 40 or 50 at Shimon's expense.

This is if there are no other workers there. If other workers are there, he hires them. He has only complaints against Shimon.

78a (Rav Nachman): He may pay others, at the expense of the retractors, up to the wages of the retractors.

Question (Rava - Beraisa): He may pay until 40 or 50 Zuz.

Answer (Rav Nachman): That is when the workers left their Klei Umnos (tools of their craft) with Reuven.

Rishonim

Rif and Rosh (6:6): If the workers did not leave their Klei Umnos with Reuven and he hired others for more than the value of what the first workers did, Reuven pays the excess.

Rosh: Rashi explains that Reuven may pay up to the wages of what the retractors already worked. Some say that he can pay double, just like 'until their value' regarding Mekach Ta'os refers to double. The Yerushalmi supports this. According to Rashi, if he already paid them for what they did, he cannot make them return it.

Hagahos Ashri: If one said 'be Mekudeshes to me for a Maneh', and he gave her a security, she is not Mekudeshes (Kidushin 8b). Since the Maneh is not here, the security takes no effect. Similarly, if one promised to give a certain amount and gave a security, there is no lien on the security, unless he was Makneh through a proper Kinyan and then gave a security. It was said in the name of R. Tam that if one gave a security on condition to give a Maneh, the receiver may hold the security until he gives it.

Rambam (Hilchos Sechirus 9:4): If Reuven's workers retracted from Davar ha'Avud, whether he hired standard workers (e.g. by the day) or Kablanim (they accepted to do the task for a set wage), the workers can retract only if Ones occurred, e.g. illness or death of a relative. If they retracted without Ones, Reuven hires others for more at the expense of the retractors or tricks them. To trick them, he says 'I hired you for one. Take two, and finish the job', and he gives to them only what he initially promised. Even if he gave to them two, he takes back the addition. To hire others at their expense, he hires other workers to avoid a loss. Whatever he pays the latter workers more than he promised to the retractors, he takes from the retractors. If he had money of the retractors, he may pay the latter workers up to 40 or 50 Zuz per day per worker, even though the worker received only three or four. This is when he cannot find other workers to finish the job for the initial wage. If such workers are available, he hires them and has only complaints against the retractors. For a standard worker, we estimate the value of what he did. If he was a Kablan, we estimate the value of what remains to be done.

Magid Mishneh: The Rashba and Ramban hold that when Reuven has their Kelim, he can hire others at their expense even if it is not Davar ha'Avud, but the price of labor rose. The Rambam explains that the Beraisa permits 40 or 50 Zuz per day per worker; this is an exaggeration (surely, one would not need to pay so much, for workers normally get three or four per day).

Rosh (2): The Ra'avad derives that one may trick workers who retract even if they did not start yet. The Reisha says 'one who hired', and we established it to be before they went to work. Also the Seifa (which permits tricking workers) says 'one who hired'! The Beraisa says 'this is if there are no other workers there.' This is obvious! Why should Reuven be allowed to use their money to pay more than is needed? It seems that this refers to tricking them. If other workers were there and he tricked the retractors, he must pay what he promised.

Poskim

Shulchan Aruch (CM 333:6): To hire others at their expense, he hires others workers to avoid a loss. Whatever he pays the latter workers more than he promised to the retractors, he takes from the retractors up to their wages.

SMA (25): The Rosh and Tur explain that if the worker was hired for two, he intends to give to Reuven the same amount from his own money if he will retract. Therefore, Reuven may hire others for four. This is like one who agreed to buy and gave a security, and the seller obligates himself to return double if he will retract. The Rambam and Shulchan Aruch cite the Gemara, but they hold like the Rosh, for they say 'whatever he pays the latter workers more than he promised to the retractors, he takes from the retractors. This connotes that he takes from their pockets, unlike Rashi who says that he can only withhold what he owed to them. Also, they do not distinguish between whether or not he paid them yet.

Shach (32): The Magid Mishneh and Beis Yosef hold that Rashi holds like the Rambam. This is wrong. I do not know why R. Yerucham holds that Rashi's opinion is primary, against R. Yerucham's Rebbi (the Rosh), especially since the Rosh's opinion is primary and the Yerushalmi supports it!

Darchei Moshe (5): Hagahos Maimoniyos (Hilchos Sechirus 9:5) says that a teacher is like a worker, unless he contracted to teach half a Sefer (or something similar). Then, he is a Kablan. The Mordechai (456) says that in either case he is not a worker who can retract in the middle of the day, for we cannot apply 'Bnei Yisrael are My slaves, not slaves of slaves.' If many, i.e. at least three, hired him, they cannot retract even before he began, for acts of the Rabim do not need a Kinyan.If there was an Arev (guarantor), even without a Kinyan the Arev is liable to pay, even if he did not yet start working (and the employer retracted). Since he cannot work elsewhere, it is as if he started.

Shulchan Aruch (ibid): If he had their money, he may pay the latter workers up to 40 or 50 Zuz per day per worker, even though the worker only received three or four.

Beis Yosef (DH v'Chosav ha'Ramban Zal): The Ramban (also Nimukei Yosef cited above) says that if one paid 100, the retractors do not pay this, for it is not a proper work contract. Workers normally get only up to 40 or 50. It seems that the Rambam disagrees. He holds that it is an exaggeration. There is no difference between 100 and 40 or 50!

SMA (26): The amount is what is normal to pay for Davar ha'Avud. This can be more or less than 40 or 50. It seems that the Rambam and Shulchan Aruch mean this, unlike the Beis Yosef and Darchei Moshe wrote.

Shach (33): The Shulchan Aruch says like the Rambam and Semag. They do not say that the workers's money was a security or that they are Makneh it. This is explicit in Tosfos (Kidushin 8b DH Maneh) and Piskei Tosfos, and the Tur connotes like this. Semag and Ri b'Rebbi Yitzchak say that it is only they did a Kinyan and stipulated that he can hire workers from it if they will retract. The Mordechai says that Rashi holds like Tosfos; this is primary.

Shach (34): The Shulchan Aruch says like the Rambam and Semag. They do not stipulate that the workers' money was their Klei Umnos. Also the Tur and R. Baruch connotes that it can be any money of theirs. The Mordechai says that Rashi holds that it is only their Klei Umnos; I disagree. Rashi merely says that this is a typical case in which workers deposit their tools with Reuven. I think that also R. Baruch explains Rashi this way, just he disagrees (and holds that the money cannot be their Klei Umnos), for if so Meshichah was done on their Kelim and they cannot retract! I say that Rashi agrees that if Meshichah was done, they cannot retract for any amount. Here they retract against Reuven's will, and he cannot force them, e.g. there is no Beis Din or they do not heed Beis Din, or we discuss a worker (not a Kablan).