What is the opposite of free as in "free of charge" (when we speak about prices)? We can add not for negation, but I am looking for a single word. 6 For free is an informal phrase used to mean "without cost or payment." These professionals were giving their time for free.

Understanding the Context

The phrase is correct; you should not use it where you are supposed to only use a formal sentence, but that doesn't make a phrase not correct.