Would this be cheeky - bed shop

[Mumof118]

I bought a mattress back in October that ended up being too hard. About 2 weeks ago I called up the shop (big company D) and asked to swap it on their 30 night comfort guarantee.

I picked another mattress that was £400 more expensive. The exchange is due next Tuesday, so I haven’t got it yet.

However, I noticed today that the new mattress has been reduced by £400! 😩

Would it be reasonable to ask them for that reduction or have I had it?

Might be important to add that the new mattress is not the store brand, but rather Sealy brand, so they would have had to order it?

No harm in asking. Is it's a big business (not a little family affair) maybe be as demanding as you can bring yourself to be.