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Although it seems very bad practice what they have done, I am afraid to say that until Contracts have been exchanged, neither party is committed to sell or buy, and either party can pull out of the transaction, without paying any compensation to the other party.
Therefore, legally, there is absolutely nothing you can claim back for your financial loss from the Developer or Agent.
I am sorry this is not the answer you were looking for, but it sets out the legal position.