GST on try party agreement for under construction property sale

dear experts,

can anyone explain how and who will pay the GST in following transaction

A builder has enter into a agreement with landowners for redevelopment of property (residential complex)

accordingly flats were allotted to landowners as per redevelopment agreement. and construction work started.

later on one land owner has sold his flat (alloted as ler agreement) to third party, and they (Builder - Landowner - Third party buyer) entered into try party agreement for the sale of this flat.

my question is who will pay the GST on this sale of under-construction flat via try party agreement.

request to kindly reply with section reference.

thanks