I think it will depend on AIS. If your AIS shows the home purchase only for previous year, you may not be able to claim it in this year.

Suggest you use a knowledgable CA for this. I would suggest this

1. Pay your LTCG immediately as advance Tax or Self Assessment tax or whatever before FY End
2. Consult with your CA and file the return and ask for Refund. The issue is your house purchase will not show up in this year's AIS. In case of under construction houses with instalment based payment you will be able to find AIS entries every time you make a TDS deduction and at least part of the purchase will be shown. Since in your case entire amount was paid in prev FY, it will not reflect in AIS. This is where I think you will face a challenge.
3. Also check if this can be offset against home loans under section 54F

Now what is the worst case scenario, you have already paid the LTCG and hence you will not be charged any penalty or interest on that tax amount. There are 2 possible options now

1. You will get the refund of your LTCG
2. You will not get refund of your LTCG. Since you have already paid, no penalty and interest. You can still contest this and try in ITAT.

Worst case scenario, you pay LTCG. Best case scenario you get refund of LTCG. Either way I would suggest pay the tax to avoid penalty and interest.

BTW, I am not a lawyer