Keep looking as I certainly wouldn't cut down the rug and redesign it based on the advice of a salesperson - especially since they have a vested interest in you buying the rug.

Such a procedure done correctly wouldn't be cheap so I am assuming the rug itself is expensive. FWIW, I have a vintage Chinese Deco rug which needed work on the fringe because someone had sewed a fringe which was worn anyway so the rug company repaired it to be the original non-fringed edge. It cost me almost $500 to have this very small repair done and the rug cleaned. The rug itself wasn't a large area rug so the amount of repair on each of the short ends wasn't that much footage but skilled labor is not cheap.

Have you considered a custom rug? That is what I wound up doing for my living room since I couldn't find the combination of color, design and size I wanted.

I have a karastan that according to the label is 12/20 but is now 12/13- someone cut it to eliminate moth damage. I can see the pattern mistake but with furniture no one else can. Would I do it with a brand new rug? Unlikely.

IMO if 5" makes a difference you ordered a rug too big for the space even if you got it right. BTW the salesperson needs to explain how that would be accomplished without total damage to the rug. get a different rug and if you want to know the right size post a to scale drawing of where it is going.

Shouldn’t there be 4 borders? Are you only planning to remove it from the short ends? Cutting off 4 would be a mistake, cutting off only 2 a complete disaster IMO. Sorry.