It is a condition that the money of the hiring be known via mutual sight or description which removes ignorance. He (saw) said:

Value, however, is not a condition in the wage just as value is not a condition in the price of a sold good. The distinction between the value and the price is that the value is what conforms to the monetary value of the thing, and is equal to it according to the measurement of those who measure. As for the price, this is that over which mutual consent occurs or greater or less (than that). It is not a condition that the wage of the employee be the value of the work because the value is not an alternative in the hiring so it is permitted for the wage to be more than the work’s value, and it is permitted to be less than the value of the work. If a person were to hire an employee for a known wage so as to mould for him a piece of gold or silver in a specific fashion, then it is permitted as he is hired for specific work and equivalence is not a condition between the wage and what he works within it from silver and gold in the weight. This is because the condition of the wage is the exchange of work, not the exchange of the domain of work which he performs. Whatever is suitable to be an alternative in sale like cash and the like is suitable to be an exchange (badal) in hiring i.e. whatever is suitable to be a price is suitable to be a wage. As for that which is not suitable to be a price in sale, it is permitted to be an exchange in hiring. For example, it is not permitted to buy an animal in return for residing in a house for a year, but it is valid to hire a garden (in exchange) for residing in a house because sale is exchanging property for property so exchanging wealth for benefit is not considered a sale. Contrary to hiring which is a contract upon a benefit for compensation, and this compensation need not necessarily be wealth but can rather be a benefit.