Is it permissible to pay in installments the value of a vow to slaughter two sheep for the poor, due to inability to fulfill it [the vow]?

Allah commanded the fulfillment of vows. A legitimate vow must be fulfilled in the manner it was vowed, unless there is a complete and irremediable inability. If the condition upon which the vow was contingent is met—which is returning to the homeland—then it is obligatory to fulfill it by slaughtering for charity. It is not permissible to deviate from it or substitute it by distributing its monetary value. It is not obligatory to fulfill it at a specific time if no time was specified, nor is it necessary to fulfill it while incapacitated. It is permissible to delay it until one becomes capable, and to slaughter the two sheep at two separate times whenever it becomes feasible.