Can an owner who is indebted to the co-ownership and who sells his units leave without paying for them?

In principal, no. Article 25 of the law on co-ownership obliges the public notary who handles the sale to ask the building-manager if any debts exist. The latter sends the public notary a “Building-manger certificate” which stipulates the debt to be deducted from the sale price. This money will be transferred to the account of the co-ownership or will remain blocked on the public notary’s account just until an agreement has been reached. (Art. 25, loi du 16 mai 1975 portant statut de la copropriété des immeubles bâtis)