Hi there, apologies for not being able to reply until now, my internet went down and it has just been repaired.
Going back to your query, your rights will very much depend on what was actually ordered and agreed in the conversation in question. If the GM ordered the correct oven and never approved a change to another model, you can argue that the seller has acted in breach of contract and also contrary to the Sale of Goods Act, which still applies to business purchases. What would matter then is whether the difference was obvious on delivery and if it was, why did you install it anyway. If the differences we subtle and not immediately apparent, you have an argument that you should not be charged anything for having installed it. On the other hand, they could argue that whilst the wrong item was delivered, you should have simply held it in the condition it was, rather than install it and thus make it difficult to resell.
If the wrong item was ordered and the manager actually approved the new oven, then your rights will obviously become more limited. You will not have an automatic right to return it or exchange it and that would only happen if the purchase terms allowed it or with the consent of the seller.
I know they say the tapes cannot be released for 30 days but you have to advise them that you reserve the right to return the item even after the 30 days, assuming they sent the wrong oven.
In any event, I suggest you do not use the oven as that could make it more difficult to return it, especially that now you know it is the incorrect one.
Does this answer tour query?