UK Property Law
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Sorry I think because yours was an old question I did not receive notification of your reply.
He would be able to sue you for breach of contract, it's a bit more complicated than just the difference between the deposit and the purchase price. He would be able to sue you for all losses flowing naturally from the loss (ie. all fees he had to pay etc) but he would be under a duty to mitigate his loss which may be arranging another sale possibly at a different price - so he would be able to sue you for the difference in price and all the expenses he had to incur in effecting the resale.
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