Proceeds from a documented sale of lawfully acquired land are lawful capital, but the requirement is proof of the whole path — how the land was acquired, what debts were secured on it, how it was sold and where the money went. The origin is not in question; the documentation is.
The land is the easy part; the paper trail behind it is the requirement
The regulation requires evidence that the investor's capital was obtained through lawful means, and USCIS reads that as a request to trace the funds back far enough to be satisfied about their origin. For farmland this can reach back decades. The couple should expect to produce the deeds and purchase documents from when each parcel was acquired, including any transfer from a parent's estate; the registered mortgages and their discharges, because a mortgage-financed purchase raises the question of how the mortgage was serviced; the sale agreement and lawyer's closing statement; and the bank records showing the net proceeds credited and later transferred.
Income tax and farm financial statements over several years demonstrate that the operation generated the means to hold and pay for the land. Where records have been lost, secondary evidence — land registry abstracts, lender confirmation letters, the lawyer's file — fills the gap. Cash transactions and undocumented family transfers are the usual weak points and should be identified early.