Ishaq bin Abdullah says that they were three hundred people, so the Prophet (peace and blessings be upon him) fixed three hundred dinars per year on them. Al-Shafi'i (may Allah have mercy on him) says: Then he made peace with the people of Najran on suits of garments (hullahs) which they paid to him; his making peace with them on something other than dinars shows that whatever they make peace upon is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18679