Just reading what you have mentioned above. The buyer and you have only signed the S&. Yes, you are protected by law that should anyone back-off from the transaction, in this case which is the buyer, you can chase for the damage.
I would say you have a good chance to win the case, but you should consult your lawyer for the particular details first. Wether you can make the claim through Small Claim Tribunal you should discuss you case with your lawyer for details, as I believe that the value of the property concern would have a good chance to be over HK$1M. Then the case would be more complicated. I have seen buyer not completing the transaction when the value of the market goes down and vender not completing the transaction when the value of the property goes up. These people are taking the chances that they would not get sue as suing require extra money to pay for the legal fees upfront. Moreover, this could also take up a long time.