Is it permissible to consider money that was spent without intention as part of a vow, and can a vow be fulfilled in installments, given that the value of the vow fluctuates between $1500 and $3500?

If you are reasonably certain that the amount of the vow (nadhr) is $1500, then you are obligated to disburse that exact amount without any increase. Intention is a prerequisite for a vow; therefore, money given as charity without the intention of fulfilling the vow does not suffice. Rather, it must be disbursed with a complete intention. It is permissible to disburse the vow in installments if no specific time was set for its disbursement and if the vow was not made to be fulfilled immediately.