Is it permissible to buy and sell devices and equipment that are not available at the time of signing the contract, or to take an advance payment or down payment for them, on the condition that they will be imported or purchased later with agreed-upon specifications, in order to avoid price increases for the customer?

If the aforementioned basic pillars are included in a single contract for the supply and installation of devices, then it is a contracting agreement, and it is not a condition for the devices to be owned by the contractor at the time of the contract. However, if the matters are separate in contracts, then the described object of sale in liability (dhimmah) is a salam sale (forward sale), and it is a condition for the entire price to be paid in advance at the contracting session. If the entire price is not paid in advance, the salam sale is not valid. As for a promise to sell, there is no harm in it, and neither party is obligated. The party who incurs damage may recourse against the other party for the actual damage incurred, and it is permissible to take a sum from the promising buyer to cover the damage, but not as earnest money.