The wikipedia entry about the Muslim conquest of Persia says:
First invasion of Mesopotamia (633)
After the Ridda Wars, a tribal chief of north eastern Arabia, Al-Muthanna ibn Haritha, raided the Persian towns in Mesopotamia (what is now Iraq). Abu Bakr was strong enough to attack the Persian Empire in the north-east and the Byzantine Empire in the north-west. There were three purposes for this conquest: 1. Along the borders between Arabia and these two great empires were numerous Arab tribes leading a nomadic life and forming a buffer-like state between the Persians and Romans. Abu Bakr hoped that these tribes might accept Islam and help their brethren in spreading it. 2. The Persian and Roman populations suffered with very high taxation laws; Abu Bakr believed that they might be persuaded to help the Muslims, who agreed to release them from the excessive tributes.
When did this agreement take place? Did it happen before the battle or after the battle?