You could ask your local council what the developer can build with your block and what he can build with the other houses but without getting yours.
Eg. If he can build up to four levels without your block, then you may end up with a four storey monstrosity blocking your light and breezes and you may lose whatever privacy you now enjoy. This would be enough for me to seriously consider selling. Your "overanxious buyer" is clearly the developer but he will be looking to get your block at the cheapest possible price, whereas you will be wanting a premium.
As devank says, you probably don't want to be plonked beside a beast of a development next door. I don't envy you the decisions you need to make. Either way, you will likely have a multi-level unit block next door. How do you feel about that? Would it be enough for you consider moving? If so, then only you can decide whether you want to negotiate a price you are happy with and that the developer is happy with.
You may decide you are happy with (just choosing random prices here) $600K because that is what it would take for you to find another house you are happy with, and/or that price is what your house is worth to you. Developer may decide he can only make his development work if he gets your place for $500K.
You can tell him "no way" but don't lose sight of the fact that any future buyer will not want to pay a premium for your house once a big block of four level units is next door. So, you could hold out for more from the developer, lose him as a buyer, and he goes ahead and builds the units anyway, and you will risk losing more due to what is now next door than what you perceive you are losing by selling to him.
Tricky...