I say, and with God's help: The obligatory amount is the nisab, not the exemption, which is what is between the two nisabs. If one possesses thirty-five camels, the obligation, which is a female camel in her first year, is only on the twenty-five, not on the total. Even if ten camels perish after the year, the obligation remains the same. It is called an exemption because the zakat is obligatory before it exists. See: Fath Bab Al-'Inayah 1: 505, and God knows best.