The actual situation is a bit more complex. Basically, the EU would argue that EU law trumps national law, and this is also in some of the contracts... however, there is no higher authority here in Germany than the consitution, some parts of which cannot be changed. Basically, if our constitutional court decides that some of the things going on in the EU are unconstitutional, and the legislative can't change those paragraphs in the constitution, then Germany would crash out of the EU. This is similar for a lot of the other member states, so there's a lot of careful maneuvring to avoid these kinds of confrontations.

One situation where this almost happened was during the discussion on Eurobonds. [0]

[0] https://www.dw.com/en/eu-launches-infringement-proceedings-a...