The Economic System of Islam by Taqiuddin an-Nabhani

In some types of hiring, it is necessary to mention only the type of work hired for, such as sewing, or driving a car to a named place, without mentioning a duration. In other types of hiring, it is necessary to mention only the duration of hiring, without mentioning the quantity of work. An example of this is to hire somebody for a month to dig a well or a canal,

which does not need a quantity defined, only that the digging should be done during this month, whether little or much. In other types, the duration and the type of work have to be mentioned, such as building a house, constructing an oil refinery and the like. So every work that needs the time period to be defined, the time period has to be mentioned,

because the nature of the hiring has to be known. Not mentioning the time duration in some works makes the hiring unknown, and if the hiring is unknown it becomes invalid. If the hiring was contracted over a certain time period such as one month or one year, then no one of the two parties is allowed to break the contract of hiring until the time period has ended. If a worker was hired for a repeated time duration, such as twenty dinars monthly, then he has to be involved in the contracted work every month and the duration must be mentioned in the hiring contract. It is not necessary that the period of hiring (i.e. the month) has to start immediately after the contract. So it is allowed to hire a person in Muharram to work in Rajab. If the duration was mentioned in the contract or it was necessary to mention it so as to remove uncertainty, then this time period has to be defined in time units such as minutes, hours, weeks, months or years.