Planning to buys used home which is barter but nowhere mentioned on Sale deed

Hello All, So let me explain situation in detail. I am planning to buy 3 bhk flat which owner has got through barter. Problem i am having he has signed sale deed on Dec 2023. In sale deed it is mentioned that he has due of 75 lac and 8 lac gst to be paid to builder. But the owner is saying he has paid all amount and he can provide receipts for same. He recently got possession letter as well but that too having Jan 2022 date, builder is saying we are giving possession letter of same date as jan 2022. Only concern that i see is if he has paid amount and has receipt of date before dec 2023 then i think its clear problem. Builder is saying they will not give no due certificate as they givr possession letter only after nothing is due, but again possession letter is jan 2022 dated and agreement which is legal document saying 75lac pending in dec 2023. Should i belive on payment receipt and old dated possession letter? What on my mind is gettingbrectification deed having payment recept attched and saying nothing is due signed by both builder and owner