The probe began on 28 June after unions representing Printemps employees alleged there were financial irregularities on the sale.
"We have turned to the prosecutor because we have discovered multiple financial arrangements in the sale specifying exorbitant commissions and intermediaries which also get exorbitant sums," Bernard Demarcq, the spokesman for the unions at the store chain, said in May, when the unions called on prosecutors in Paris to investigate the sale.
He added at the time that the sale should be probed "for possible corruption, breach of trust, money laundering and tax fraud."
At the end of the preliminary probe, prosecutors can decide to throw out the case, or open a full-scale judicial enquiry and appoint an investigating judge.
Qatari investors, using a Luxembourg firm called Disa, bought the department store in April. It was previously owned by Deutsche Bank and the Italian Borletti Group.
The value of the same was not disclosed, but some estimates put the deal at around 1.6 billion euros.
The sale was approved by competition regulators on 22 July.
Unions claim the new owners aim to sack 226 out of around 3,400 employees.
Around 10 percent of Qatari foreign investments are in France, including the Paris Saint-Germain football club and a small holding in energy giant Total.
Le Printemps has a network of 16 stores, including its flagship store on Boulevard Haussmann in Paris.
Management at Le Printemps declined to comment.