Is it permissible to give the value of two vowed salaries in monthly installments as aid to a needy family, or must it be given as a single lump sum?

A vow to donate two salaries is a vow of obedience that must be fulfilled. If the intention was to donate the first two salaries, then one must hasten to do so. If the intention was to donate two salaries without specifying which ones, then it is permissible to pay them in reasonable installments that benefit those in need.