Is it obligatory to fulfill a vow to build a wall for a neighbor if its cost significantly exceeds the amount intended?

A conditional vow (Nathr Mu'allaq) is disliked, but it is obligatory to fulfill it. If you intended to spend a specific amount for building the wall, then only that amount is obligatory upon you. However, if you intended to build the wall regardless of its cost, then building it is obligatory upon you. It is not permissible to deviate from a vow except due to inability or harm. If you are completely unable (to fulfill it), its expiation is the expiation of an oath (kaffarat yameen).