By reconstructing the history of the land as well as the sale. Title records, the original purchase documents, the discharged mortgage, probate or transfer papers for the inherited portion, filed tax returns reporting the gain, the statement of adjustments and the bank trail from closing to the enterprise together answer both source and path.
Two questions, asked about every dollar
Source asks where the money came from; path asks how it travelled from there to the investment. A farmland sale is usually strong on source and weak on path unless it is documented deliberately. For the portion Gurpreet bought, the file needs the historical purchase agreement, the transfer, the mortgage and its discharge, and enough of a record to show the mortgage was serviced from farming income.
For the inherited portion it needs the will or the estate documents and the transfer that followed. The sale itself needs the agreement, the statement of adjustments, the lawyer's trust ledger and the deposit into his account. Tax returns reporting the capital gain corroborate the whole chain and are among the most useful documents available.
Currency conversion and any transfer through a third party must also be shown, since an unexplained intermediate account is the most common reason a well-sourced file stalls.