Moldovan returned to Nantes at the start of October and impressed sufficiently during his trial spell with the club.
The experienced forward only left Nantes in the summer to join Al-Wahda but cancelled his contract with the Middle East side due to family reasons.
Les Canaris had been keen to use Moldovan immediately but Fifa have blocked the club's request and they must now wait until the New Year.
Moldovan will be allowed to play in friendly fixtures in the interim and will pen a six-month contract when the transfer window reopens.
Meanwhile, Nantes have handed a trial to Estonian full back Teet Allas, who plays for FC Flora in his homeland.
Allas will stay at La Beaujoire-Louis-Fonteneau until the weekend in a bid to impress coach Loic Amisse.
