Is it permissible to agree with another craftsman to carry out a work previously agreed upon with a first craftsman, if the second will complete the work at the same price and in a shorter time?

The preponderant view is that it is permissible to add the ijarah (leasing/hiring) contract to a future time, and this is the madhhab of the majority of scholars. It is not a condition that the ijarah period immediately follows the contract. As for ijarah for a service to be rendered (في الذمة), it is permissible to add it to a future time by agreement of scholars. Once an ijarah contract is concluded, it is binding on both parties, and neither party has the right to unilaterally terminate it without the consent of the other, except through mutual cancellation (iqalah). If the contract is for both work and materials, it is a manufacturing contract (istisna'), which is among the binding contracts, and its ruling is similar to that of ijarah.