I suggest you issue a notice/legal letter clarifying the return of the money to Mr.K along with a cross cheque/DD for the said amount in his favour. Since there was no agreement signed, you are free to conduct sale of the property for which you hold a clear title. From what you've mentioned, in my opinion Mr. K has accrued no right/title/interest in the property and thus cannot legally stop you from conducting sale of the property.
Best of luck!
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