Question: A man said to another, “Do such-and-such, and I will pay you,” without mentioning how much he would pay. The requested act was done, and the man gave him some money; he was displeased, and a quarrel arose. What is required?
Answer: If the two men did not come to an agreement or settlement on a specified amount, then the man must pay the other the customary wage (ujrat al-mithl): one looks at what work was done and pays according to it. This wage is defective [invalid], because the amount is unknown.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2