Is it permissible to pay the vowed offering collectively for past and future months, given that the one who made the vow pledged to donate the excess of his salary over 5000 Riyals to a specific charitable cause, and he was accepted for a job with a salary of 5500 Riyals?

If you specify a particular area of charity for your vow to be fulfilled, it is permissible to delay the matter until sufficient funds are collected, because fulfilling a vow is not obligatory immediately unless the wording necessitates it. And there is no harm in expediting the remaining five months of the year.