I've got our UR5e package and the cable fitted to it, but no extra pendant cable, and the seller has sent a contents sheet with the spare-kit line missing even though it's on our accepted quote. They want me to sign the sheet before discussing the shortage. I'm not signing that as complete.
Send back a list of what you actually received and identify the spare against the accepted quote; you can acknowledge the delivered hardware without agreeing that the missing item has disappeared from the order.