Is selling a farm to Christians—especially with their potential intent to increase their presence in a Christian-majority area where prices are rising—considered a dereliction of religious duty, even if the sale generates significant financial gains for the seller?

You are not permitted to sell the farm to those whom you know or strongly suspect harbor malicious intentions against Muslims, because that would constitute assisting in sin and aggression, which are forbidden by Islamic law. The interest of religion must be prioritized over personal interest. Whoever gives up something for the sake of Allah, Allah will compensate him with something better and grant him victory.