New Zealand Law
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It would be difficult to give definite answers on this without seeing all the documents. From your description you appear to have an option for the first six months together. The contract to buy appears to be enforceable as well, so if he changed his mind, you can probably insist he sells. But I have seen the fine print so can only make general comments.
As for bailing out of the contract, he appears to be hinting he may want one of the better offers, and if you are uncertain about proceeding anyway, you should set up a meeting to discuss these issues, probably with your lawyer present. That will then sort out what you both want out of this transaction