Long story short:
I am looking to do CLT-CDG-CLT on 9/14-19. There is only 1 flight each day, operated by AA. When looking at fares for an coach ticket, the price of a BA ticket is $1087 vs. the price of an AA fare (for the exact same flight) is $1805!!

I have an unused AA tickets worth $800 (that was for a domestic flight that I had to cancel). I really want to apply that unused ticket credit to that flight to paris as it is expiring next month.

Can I apply those funds to a BA tickets as it is an AA-operated flight?
And out of curiosity, why such a difference in fares???