I think this is the relevant bit of law, point (e) about the conclusion of contracts being subject to other parties, as this covers the conclusion of employment contracts, among other things.

1. The following shall be prohibited as incompatible with the common market: all agreements between undertakings, decisions by associations of undertakings and concerted practices which may affect trade between Member States and which have as their object or effect the prevention, restriction or distortion of competition within the common market, and in particular those that:

(a) Directly or indirectly fix purchase or selling prices or any other trading conditions

(b) Limit or control production, markets, technical development, or investment

(c) Share markets or sources of supply

(d) Apply dissimilar conditions to equivalent transactions with other trading parties, thereby placing them at a competitive disadvantage;

(e) Make the conclusion of contracts subject to acceptance by the other parties of supplementary obligations that, by their nature or according to commercial usage, have no connection with the subject of such contracts

http://eur-lex.europa.eu/Notice.do?val=262491:cs&lang=en&lis...