The Bluebirds have issued a series of questions which they want comprehensively answered before they pay the Ligue 1 team any money relating to the deal.
Cardiff have written to Nantes after club chairman Mehmet Dalman implied there were some inconsistencies and concerns surrounding aspects of the transfer.
The Argentinian striker died when the single-engine plane taking him back to Wales from France crashed into the sea off Alderney in the Channel Islands on January 21.
His funeral was held in his hometown of Progreso on Saturday. Pilot David Ibbotson is still missing.
The Sun has an exclusive report which details the ten key questions to which the English Premier League side are demanding answers.
• Who arranged the flight and why did that person put Sala on a single-engine plane at night in difficult conditions, without adequate emergency apparatus?
• Who took the decision to put Sala on the plane?
• Why won’t the owner of the aircraft come forward?
• Was the plane licenced to take commercial passengers?
• Was Sala third-party owned?
On February 6, Nantes demanded Cardiff pay the first instalment of the transfer fee and while the Welsh side are willing to honour the payment, they want the western French side to clarify certain details of the agreement.
The Sun claims that Cardiff chiefs had sent Sala a text message with details of British Airways flight options - normal procedure with any new overseas signing - but they are baffled as to why the striker took the small aircraft instead.
There are also questions over agent Willie McKay’s role, and why he didn’t spend around £5,000 to hire a private jet for the player. The agent also admitted falsely claiming in the media that Everton and West Ham were keen on Sala, boosting the player’s transfer fee.
Cardiff are also urgently trying to ascertain if Nantes may have flouted strict Fifa rules about third-party ownership, and have pressed the French side to provide a breakdown detailing commissions due to various intermediaries.