Ask for the delivered assembly's identity and dimensional comparison against the drawing they confirmed. Your fixture designer needs that actual difference, not a promise that it is generally usable somewhere.
Who pays for that bracket and its assessment? The seller can't call the configuration equivalent and quietly leave your workshop doing the design work needed to make it usable.
You can compare the modified arrangement if you choose, but keep it separate from supplying the confirmed configuration. Its new bracket, installation work and relevant checks need an actual proposal before it becomes the assumed solution.
Then ask them to identify what they would supply and how the delivered package would be returned. An agreement to discuss replacement is not yet a collection arrangement.