In the second, it is not clear whether they were concerned about past or present salaries. Context is need to answer that question.
It is quite normal to state a subordinate clause in the past when the main clause is in the past, but it must be clear that the subordinate clause is not necessarily dealing with past time. The only time it is wrong to do that is when we are dealing with an unchangeable condition.
In 1492, Columbus knew that the world is round (not was round).
That said, a subordinate clause can be clearer in the present tense if it is referring to present time.