Nice chairman Jean-Pierre Riviere claims there is no guarantee the club will sign summer target Yannick Djalo in January.
The Ligue 1 outfit thought they had put a deal in place to bring the forward to France during the last transfer window.
An agreement was reached with Sporting Lisbon and the relevant paperwork was signed.
They were, however, unable to get the documents processed before the deadline and the proposed move was blocked.
It was expected that Nice would return for the Portugal international in the New Year, but Riviere claims no decision has been taken as yet regarding a possible switch.
"We have been left disappointed," he told L'Equipe.
"It is a shame for both us and him that a deal has not happened.
"We wanted him now and nothing has been set up for the future. There is no agreement to sign him during the next transfer window."