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You mean he has paid when it is not due?
The problem is that if you just retain it then the deposit scheme will almost certainly rule in his favour and release the deposit.
You can always recover the additional amount by suing him though. If it is under £10k then it would be a small claims court sum which would mean it is cheap and quick to issue against him.
In principle you could try retaining the amount and inviting him to sue if he thinks he is not liable but I wouldn't recommend it here.
Can I clarify anything for you?