86 year old client sold agricultural land in cash for less than stated in land records

There are more than one approach to handle this situation.

you have to compute the capital gain on land by taking 5L as sale price. but would you please check if its written in the sale deed that your client has received the money in cash. if yes, then penalty provisions might attract.

the problem lies with remaining 10 lacs deposited in bank. you have to show as if that was his accumulated savings or as gift receipts or as other income or as sale of some other assets(it will be best if such asset is personal asset on which capital gain tax does not arise) or a combination of these ideas.

since cash deposit exceeds 10lacs, the transaction will be reported to department. you have to anyhow show total 15 receipts in itr. if they dont issue 143(2) then i think they wont be able to issue 148 in later years because you have explained the source of cash deposit of 15l in the itr. only if you had not shown the source of such cash would they issue 148. even if you showed it wrongly they cant scrutinise it during 148. it had to be done in 143(2).
hence file the itr and wait until september 30(due date of 143(2))

Make 3-5 year financials and ensure opening cash in hand is more than 10 lakhs as guided by Veerendar Singh ji.