Taking as evidence what we have narrated in the Book of Sales from the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) forbade the sale involving deception (gharar); and hire contracts are a type of sale, and the lack of knowledge (of anything) in them is deception.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / Q11651
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "A man should not bargain over his brother's bargain, nor propose over his brother's proposal, nor bid (to inflate the price), nor transact sales by throwing a stone; and whoever hires someone should settle his wage with him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11651
It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) "forbade hiring a worker, that is, until he makes his wage clear to him."
It is mursal (with a break in the chain) between Ibrahim and Abu Sa'id.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11652
Sayyiduna Awf bin Malik (may Allah be pleased with him) says: We went on a campaign, and our commander was Sayyiduna Amr bin al-As (may Allah be pleased with him), and among us were Sayyiduna Umar bin al-Khattab and Sayyiduna Abu Ubaydah (may Allah be pleased with them both). We were struck by severe hunger. I went out to look for some means of living, and I found some people who wanted to slaughter a camel. I said: If you wish, I will do the slaughtering and the work for you, and you give me some of it. I did so, and they gave me some of it, and I cooked it. Then I came to Sayyiduna Umar (may Allah be pleased with him), and he asked: Where is this from? I told him, and he said: You have hastened your wage, and he refused to eat it. Then I told Sayyiduna Abu Ubaydah (may Allah be pleased with him), and he said the same as Sayyiduna Umar (may Allah be pleased with him) had said, and refused to eat it. Seeing this, I left it. He said: Then we were granted a victory, and they sent me as a courier with its news, so I came to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "The man of the camel," and he said nothing more to me.
The Shaykh said: This shows that the wage was unknown, and that it was owed as a liability tied to a specific thing, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11653
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him): "Give the worker his due before his sweat dries, and inform him of his wage while he is at his work."
This is extremely weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11654
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he said: I grew up an orphan and I emigrated as a poor man, and I was a hired worker for Ibn Affan and the daughter of Ghazwan, in return for the food of my stomach and a turn at riding. I would gather firewood for them when they camped, and chant the camel-song for them when they travelled.
So all praise is for Allah, who made the religion a mainstay and made Abu Hurayrah (may Allah be pleased with him) an imam.
There is nothing in this to show that the Prophet (peace and blessings be upon him) knew of it and let it stand, and it is possible that this was an arrangement between them by way of mutual consent, not by way of a contract.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11655
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) saw no harm in a man giving a garment to another man and saying: "Sell it for such-and-such; whatever you get above that is yours."
This too would be by way of mutual consent, not by way of a binding contract, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11656