The agent was wrong and it is a requirement of the contract that the 10% be paid on exchange, at the time of the auction, or the purchaser is in breach of the contract.
No they cannot put a caveat on the purchasers house but they can threaten to terminate for non compliance of the contract.
They are in default.
The issue will raise its head when settlement comes around and if they do not settle and you serve a notice to complete and they fail to to comply and you terminate then you have to sue them for the money.
Until settlement there is nothing you can do apart from telling them they are in default and seek to terminate if they do not pay the 10%.
If they refuse to pay and then then do not settle you can sue for the whole lot with interest as per the contract.
But at present all you can do it threaten with with termination because they have not paid the deposit and no compiled with it.
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