Moq’ing methods where Expression are passed in as parameters

It is probably correct that only an Expression with the exactly same structure (and literal values) will match. I suggest that you use the overload of Returns() that lets you use the parameters the mock is called with:

repoMock.Setup(moq => moq.FindBy(It.IsAny<Expression<Func<Company, bool>>>())
        .Returns((Expression<Func<Company, bool>> predicate) => ...);

In ..., you can use predicate to return the matching companies (and maybe even throw an exception if the matching companies isn’t what you expected). Not very pretty, but I think it will work.