As has been written many times, there are always two sides to the coin here. On the one hand, a fixed price is a fixed price. On the other hand, what happens if the company stops construction and you might then have to take legal action? Not to mention the construction delays. Instead of exchanging emails back and forth, I would ask for a personal conversation to clarify it objectively (without emotional outbursts). Perhaps a 50/50 solution would be possible, which would be cheaper than construction delays/lawyers.