What is FC Barcelona’s Negreira case, and what are the charges?

What happened
Real Madrid accuse Barcelona of paying former refereeing official Jose Maria Enriquez Negreira between 2001 and 2018.
Summary assembled by rule from the sources below

Real Madrid accuse Barcelona of paying former refereeing official Jose Maria Enriquez Negreira between 2001 and 2018.
Summary assembled by rule from the sources below