That is not the case, when a buyer puts a down payment for the purchase its usually kept by the lawyer, in case the buyer takes that 10% or 15% he typically uses that money to pay the agents and any other utility charges and fees that may apply.
The buyer then has a defined period in which he is to obtain that mortgage if he fails to do so then you either need o to pay him back the advance money and run after the agents to get your commission refunded since there was no sale, or you get to keep that advance and the agents get to keep their commission but this only happened in the case when the buyer is at fault and not the bank etc...
doing this period the buyer isn't the owner of the property and will not be the owner of this property until you sign the final declaration that you received the full sales amount and you agree to hand over the property and the registration.
during this time there is merely a notice in the land-registry to prevent you the seller from selling to another...
i hope this was helpfull