Hi Mamona
Interesting discussion. So I thought I would read the IPSAS Final Pronouncement 2017 "Financial Reporting Under the Cash Basis of Accounting." That statement makes a defining line at the point when an organization has or does not have control of cash - so not having control means it is no longer your cash. Given that, if you write a check and mail it you no longer have control over it. It is now a recordable payment of cash.
On the cash receipt side, this is even clearer. The statement says 1.2.7 "Amounts deposited in the bank account of an entity are controlled by that entity." And they said "amounts" not "pure cash" so a check would qualify. And consider, if the auditor applied the "a check is not cash" rule to deposits, then you would have a nightmare and have to see when each check given to you for deposit actually cleared before you could record it.
So, I say resist this auditor and save your sanity!
Regards
Kat