Imam al-Shafi'i (may Allah have mercy on him) said: Giving the expiation by feeding before breaking the oath will suffice him. This is Allah's right over the wealth and persons of (His) servants; if they fulfil the right before its time comes, it will suffice. The basis of this is that the Prophet (peace and blessings be upon him) took the zakah from Hadrat Abbas (may Allah be pleased with him) before the year had passed, and the Muslims pay sadaqat al-fitr before Eid al-Fitr; so all (such) rights were deduced by analogy with this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19965