I am not sure if this is the right forum to ask, but this is the forum I am most familiar with.

I am flying United on the outbound, and AA on the in-bound for an upcoming international trip traveling in Business Class. I have no control over the ticket as it is purchased through a third party and paid by another party. I just show up and fly with a paper receipt and e-confirmation.

Based from what I have been told, if I want to fly UA on the in-bound flight without changing the ticket, I would need to get AA to endorse the ticket to UA. Is this still correct? Is there anyway UA would honor the ticket without me going to AA first?

Thanks in advance for any pointers.