Okay...I'm confused. If US Airways endorsed the ticket over to AC or wrote out a FIM for J class, then they would be billed for the J class fare that the airlines negotiate among themselves.

In that case, AC should have been glad they were getting some revenue for a unsold J seat.

If US Airways is willing to eat the cost to accomodate their upgraded customer on another carrier and pay them for it, then why should AC care?

Next time, ask US to reissue the e-ticket on AC. I'm not sure if they did that or what exactly they gave you.