The problem with your question is to define trade and quid pro quo.
- Alice is a friend of Bob.
- Bob visits Alice to watch a movie together and he brings an apple pie.
- A week later, Alice visits Bob to watch a movie together and she brings a sixpack of beer.
Is that a quid pro quo or just friends who see that the effort of their joint time is shared equitable?
Translated to the EU, if a foreign power were to bribe an EU member government out of the blue sky to do something like blockading an EU decision, against the common EU interest, one might try to argue that this goes against the fuzzy principles laid down in the TEU. But that's not what is likely to happen. If an EU state has special animosity or amity to a non-EU state, and if that colors their official policies, is that bribery or standing for established political traditions?
For instance, Article 32 TEU requires that "Member States shall show mutual solidarity." Article 24 TEU requires that "the Member States shall support the Union's external and security policy actively and unreservedly in a spirit of loyalty and mutual solidarity and shall comply with the Union's action in this area."
As principles, these are fine, but they do not spell out when a foreign investment deal becomes to good to be true, or when a traditional friendship with a non-EU country becomes not showing solidarity.