If rava says that charity must be paid out right away as a vow, then why earlier does the gemorah list charity as one of the items that can wait till all 3 regalim?
Tosfos (4a DH Tzedakos) and the Rishonim (ibid.) ask your question. Tosfos answers that one transgresses Bal T'acher immediately for Tzedakah only if there are paupers around and one refrains from giving them. If there are no local paupers, one has three Regalim to seek some out.