Simple, and almost entirely ineffective. Unless you commit substantial resources to policing the area (keeping in mind that the entire Amazon rainforest covers over two million square miles, though the purchase almost certainly wouldn't be the entire thing), then you'll have plenty of people crossing the line, going in, clearing sections of it, and planting crops. Because they need money to survive and that is a comparatively easy path to money (need/want for money is why parts of it are currently getting burned/leveled - it's not like people are cutting it down for fun). You'd either need substantial enforcement around the perimeter, or, more usefully, work to improve the opportunities to make a living for everyone surrounding this purchased area, so they don't feel the need to go burn down parts of it.