Can you use a completed core requirement class to also satisfy a class requirement for a major or due you need to pay for a different course ? Thank you !

If I am understanding your question correctly, you cannot do what you're proposing. A course can only satisfy one requirement. So a history major couldn't use the same HIST course towards their US History requirement and one of their major coursework requirements. The only exceptions to this are the ICD requirement, the writing intensive course requirements, a minor, etc. Talk to your academic advisor for more details about your specific major.