My fiancé, who lives in Accra, Ghana, has been told she must provide a $6000 declaration fee to the embassy before they will let her travel to the US on a tourist visa. The money will be given to her along with her passport, visa, and plane ticket, which I have already purchased for her. She was originally told the fee was $1000, which I also paid. On the same day she paid the embassy with the money I sent to her, they told her they had made a mistake and the fee was actually $6000. She has tried to negotiate this fee down with no success. Is this $6000 declaration fee legitimate?

This is a well known SCAM!

Have you physically met this person?
They attempt to get as much money as possible from their marks and then disappear.
Refer to this sticky: https://www.askmehelpdesk.com/visas-passports/ghana-romance-scams-442160.html

I think your friend is trying to scam you. In my opinion

Scam!!