According to BBC Wales, Nantes have given Cardiff a 10-day ultimatum over the first payment of between five and six million euros.
A source at Cardiff told British media that the Welsh club will honour the contract but only once they have clarified "all the facts." Cardiff are said to be "surprised" with the timing of the demand so soon after the tragedy.
There appears to be considerable tension between the clubs. Regional newspaper Ouest-France quoted an anonymous source at Nantes calling Cardiff's attitude "hypocritical," accusing them of "wanting to buy time."