Dear Querist,
Practically, if you have documentary evidence proving payment of X (bank payments, receipts, agreement, etc.), you may claim X as the cost of acquisition.
However, if a part of the amount was paid in cash and there is no reliable evidence, it would be difficult to substantiate that amount before the Income Tax Department. In such a case, your CA may have to consider only the amount that can be properly supported by documents.
For a more detailed review of your case, you may book a phone consultation.
CA Shubham Goyal