Actually i am confused, i think no one can sell large amount of Bitcoin at discount. Why he will losser. He can sell his Bitcoin on exchanger easily. He can also sell on Binance by using p2p. I think this information has some mistake. Because there are huge Exchanger available and there he can easily sell bitcoin there without any hassle. I am thinking about another risk, like scam risk. Please be more careful about this kinds of transaction. And my strong advice please use exchanger which will more secure to buy and sell this kinds of big transaction.
I think he is pertaining to an OTC transaction which typically the buyer offers a bulk purchase on a slightly discounted price. I believe Binance has this kind of feature for institutional investors that wants to enter the crypto market without pumping the market price.
This kind of transaction are for institutional and not for the normal whales. We are talking about multi million to billion investment here. But I doubt that the OP client is one of them since institutional investors will surely purchased on exchange OTC to make ti legal.