In messages giving unique insight into the vast figures and politics behind a Premier League transfer, McKay also suggests the player's mother could claim £1million if he agreed to sign from Nantes. McKay then tells Sala that Neil Warnock has "fallen in love with you" because the Argentinian is a "typical English centre-forward, like Drogba or Alan Shearer".
The email was sent two weeks before the 28-year-old is believed to have died after the single-engined plane he was travelling in crashed in the English Channel. McKay, whose son Mark was an official intermediary, told Telegraph Sport he had released his personal message to Sala to set the record straight.
McKay admits making arrangements for the doomed flight, having told his usual pilot David Henderson: "Dave, I need a plane." However, unbeknownst to McKay, part-time engineer Dave Ibbotson, 59, was at the controls when the Piper Malibu went missing on January 21 after leaving Nantes for Cardiff.
In the prior email, McKay told Sala how Cardiff "will pay you around £50,000-per-week, plus bonuses, on a contract of four to five years. And whoever accompanies you, whether it is your agent or your mother, who, according to English law, can represent you, could claim around £1m from the deal."
He added: "It is said that you did not want to go to Cardiff. It is probably our fault because we have said in the media that other clubs like West Ham, Everton, etc were interested in order to create an interest around you."
McKay suggests he can eventually get Sala a move to "Manchester, Liverpool and Chelsea" should he sign for Cardiff. He wrote on Jan 8: "We are not interested in personal affairs; finances, holidays, baby-sitting, that is not our business."
McKay referenced the multi-million pound deals that his family had been involved in, adding: "We make transfers! More than 600 until today, from Didier Drogba, to (Nicolas) Anelka, (Dimitri) Payet, (Jean Michael) Seri, (André-Frank Zambo) Anguissa. Let us introduce you to the way we operate and how we came to this Cardiff City saga. We work for clubs in France, and for players who want to be transferred to England. As far as you are concerned, we have talked to all the clubs, including Manchester, Chelsea, Liverpool. We think you could end up in such clubs.
"We approached Nantes, as we do with many players in other clubs, to obtain the mandate of sale. We are not preventing you from working with another agent, but most players are very satisfied with our mediation. We do not say 'we are like a father to a son to our players'. No, if you had not been a footballer, these people would not be interested in you. In the end they are only interested in the money. What we all want a lot of, of course. That's why we like to work with just the clubs. No sentiment, we're just doing business."
Referring specifically to the Cardiff City deal, Mr McKay wrote: "My experience says: if you do not go for the Premier League now, you will regret it.
"It's been said that Cardiff does not interest you. But that is probably our mistake, because we 'leaked' in the media that other clubs like West Ham and Everton want you. Just to stimulate the interest in you.
"That is how we work and that may be misinterpreted by the player. But without that 'buzz' nobody would know you. Because, honestly, nobody follows the French competition [Ligue 1]."
The emails were initially published in French newspaper L'Equipe. McKay told Telegraph Sport: "I sent the emails to L'Equipe because I want the truth to come out. This is out how it works."
He said his family had been left "devastated" by events over the last week. McKay described how he arranged the flight.
"I said to Dave [Henderson], I need a plane. He said no problem. He paid the landing fees and fuel."
Crash investigators are poised to launch a sea-bed search after indicating the single-engined aircraft came down somewhere off the coast of Surtainville, on the Cotentin Peninsula. Cushions from two seats believed to belong to the were discovered on a beach about 20 miles from the last known position of the aircraft.
According to McKay, Henderson had to pay for Ibbotson's hotel in France, and for the airline fuel. Ibbotson, who had written online that he was a bit "rusty" before the flight, had reportedly told Henderson that he had lost his credit card.