Is it permissible for one who is obligated by a vow to fulfill it with two goat kids instead of a ram, especially if he does not possess the price of the ram, which is 1800 pounds, and he only has 1450 pounds?

A conditional vow is disliked, but if the condition upon which the vow was made is met, then it becomes obligatory to fulfill it in the manner in which it was vowed. It is not permissible to substitute it with something else unless that something else is better. What you did is not sufficient as long as a ram is better than a kid (goat), and the vow remains an obligation upon your mother, and she must fulfill it when she is able. If she is unable, then she must offer an expiation for breaking an oath (feeding ten poor people or clothing them, or freeing a slave; and if she cannot find any of these, then fasting for three days).