NEED THE GUIDANCE OF LEARNED LCI MEMBERS

Sir, One of my client intending to purchase a property has entered into an sale agreement with one Mr. X. While the above said sale agreement was in subsistence Mr. X has executed a sale deed in respect of the said property to a third party. Subsequently the said Mr. X has again executed a sale deed in favour of my client ? Now the said third party is claims right based on the sale deed. My client insists that since he is having sale agreement the sale made during the subsistence of the sale agreement is void ? Learned LCI members advise how to proceed. Kindly provide me citations in this regard so to proceed in the matter.