It is more on contractual requirement and process. We do have an agreement (fixed price) and list of approved brand we can choose and propose.
If that's what has been agreed to (ie,
what the contract says), then everyone should comply - unless someone has the funds required to insist on Change Orders!
On the other hand, surely the end user can propose
alternatives? Then it is up to you to agree or disagree as appropriate. Just as long as any such proposals are made in a timely manner (that is, before the selected item has already been approved for purchase). Who knows, they may even suggest something that costs you less!
Where have your specs (specifications) come from, by the way?
Meanwhile, here's another nice phrase that may sometimes be useful:- "Equal to, or better than".