I'm going to use US law terms, but I believe the same theory would apply for practical purposes. Developer pays you half of the price at closing, and signs a promissory note for the other half, due and payable in one year's time. The note would be secured by what we'd call a deed of trust/mortgage/security deed that appears of record in the public registry.
Agree with Brian.....Also, assuming that you sell the property at a value that is 100% or more of it's market value, the property (which would be the collateral in this deal-Brian's comment above about being secured by the appropriate deed of your locale (I'm in the US, as well)) is worth twice the original loan amount (Since you are loaning 50% of the value of the property) So, there is value there and you're loan is safe in that respect. Always make sure that any loan you extend can be comfortably secured by the value of the collateral, which should be much greater than your loan amount, depending on your level of risk.
Make sure you are engaging an attorney or similar who is capable of writing these docs, your note is secured against the property, and that the appropriate documents are recorded. Also, you'll most likely want to make sure that your note is in the 1st lien position, because if there are issues with successfully completing the development and you need to recoup your investment, your debt will be paid first.