What are the requirements regarding the specification of the service period in a Mukataba contract?

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It is required that the service period be known, just as it is required in an Ijarah contract. If a man contracts for service for one month and a Dinar, the contract is valid without explicitly stating the month or that the service immediately follows the contract, as generality implies that. If the month specified does not immediately follow the contract, such as contracting in Muharram for service in Rajab and a Dinar, the contract is also valid, analogous to leasing a house for the month of Rajab while concluding the contract in Muharram.