Even today, cash manipulation happens in more than 90% cases of the property sales. The registry value of the property is shown below the actual deal price and rest of the payment is made in cash. No government measure can catch these tax evasion transactions if seller is clever enough to maintain the secrecy of cash received.
You can certainly claim this digit as 99.
For example, the valuation of one property is 50L. The bank itself values this by sending valuers to the site. Now dastavej (property-owning document) will be of mere 20L (15 in most of the cases) which means that the seller has sold his property at 20L so the seller will have to pay less tax on capital gain and he will forcibly demand 30L in cash from the buyer otherwise, he would not sell it. This is "extremely common" in property cases.
If the government raid just one place and filter such brokers and sellers, oh man! this can be turned out to multi-billion tax fraud.
