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