Gst on sale of flat

We recently purchased a flat in an apartment. Sale value is 47lakhs.
Out of 47
1. We paid in CASH 12lakhs in may2021.
2. We paid through bank transfer 5lakhs on 21st July 2021.
3. We took a loan of 27 lakhs which was sanctioned on 25th August 2021.
4. We paid in cash 3 lakhs today i.e. on 7th SEP 2021.

COMPLETION CERTIFICATE WAS ISSUED ON 19TH JULY 2021.

Registration value 25lakhs. My seller is asking for GST @ 5% on the value of 32 lakhs(i.e. 5lakhs paid through bank plus 27lakhs through loan)

So I said him GST is not applicable as both 5 lakhs and 27 lakhs is paid after issuance of Completion certificate. But he is saying it's chargeable as he himself is both landowner and builder too. I'm I correct or he?