Senior sub vs. outside office subs

I’m the senior sub in my 3 route office. Every Friday, I’ve been doing R1 since early August. It’s not my matrix route, but the routes sub hasn’t been here in months. The other sub we have for our other route, was hired and is now transferring to another office but still on our books and has never even done a full route by himself in our office so I’m the only sub that’s available in my office. The only time I haven’t been doing R1 is if I request off for some reason. I didn’t do it during prime time because my postmaster asked me if I’d rather do a parcel run on Fridays instead and he’d use another sub in another office that knows the route and is pretty fast and I agreed that was fine because I would rather do the parcel runs.

Jump to this morning, my postmaster told me he already booked the same sub from the other office to do the route next Friday because he’d rather have him because he’s “faster” and it’s a day after the holiday and will be heavy. I’m available to work and I’ve always worked days after holidays if needed, and there’s never any issues? I’ve never missed the truck or don’t come back before the evaluation time. Yes this other sub is really fast but he’s not even from my office and I am available to work and I feel like he’s taking one of my work days away that I’ve been working for no reason, is this allowed?