Taxability of sale of under-construction property?

A property has been booked (Feb 2015) by giving advance payment as and when requested. The said property is under construction and would be complete by Feb 2018. 80% advance amount has been paid to company. I wish to know the taxability of transaction if the said property is sold to a third person before completion of property (say in sep 2017) and the 20% amount being due. Would it be liable to capital gain (Short term in this case) as the right to property is being transferred? Kindly clarify in the light of relevant provisions.