Is it permissible to purchase a phone in installments with the intention of not paying the full amount and taking advantage of the contract's termination clause, keeping the device upon leaving the country?

Firstly, it is permissible to include a clause in the contract stating exemption from paying the remaining installments if the buyer leaves the country. This is permissible because it falls under the category of a conditional waiver of debt, and the waiver of an unknown amount is valid according to the most authoritative opinion.

Secondly, if the seller did not stipulate that the waiver is exclusive to those compelled to travel, then there is no harm in purchasing without informing him, though it is preferable to inform him that you will return before two years. However, if he stipulated that the waiver is for cases of necessity, or if the intended reason was unknown to the buyer at the time of purchase, then it is obligatory to inform him of the truth of the matter. It is more cautious to inform the seller of the true situation so that he enters into the contract with full awareness.