The Premier League club received a legal letter on Tuesday insisting that they cough up the first instalment of the club record fee or they face being taken to court.
The formal notice came just a day before recovery teams retrieved a body from the wreckage of Sala's doomed flight to Wales which was discovered in the Channel.
French club Nantes say they are still waiting for the funds to arrive which had been due to be paid by January 26.
That was the initial date agreed upon in the deal which took the 28-year-old striker to Cardiff.
At the time the Premier League club were involved in a desperate search to find any trace of Sala and pilot Dave Ibbotson after their flight to Wales disappeared on January 21.
But Nantes believe they have given Cardiff enough time to get their affairs in order and are now calling for them to meet the terms of their contract.
Though identification of the body has yet to even take place, the Ligue One side are keen to get their own accounts in order.
Cardiff have insisted all along that they are liable to pay up the whole transfer fee, and maintain they will do so.
But they believe they have a duty of care to the family of Sala first, trying to help establish what happened with the Piper Malibu plan that ditched en-route from France to Wales.
The family are currently based in Nantes but his sister travelled to Cardiff last week to meet with club officials and collect some of his belongings.
The club's belief is that they should not be dealing with business matters while one of the two is still to be found and no funeral has been held.
They did manage to register the transfer with the Welsh FA before the plane went missing, meaning their £16m insurance policy against the player was activated properly.
But they still also face having pay up some of his salary which was not covered by insurance to the family of Sala.
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