If it's concluded then it's legally binding and the deal has been done, but don't forget the finance people have to sign to accept the deal....have they actually signed, it is the weekend.? !
If on the contrary the deal has gone through, then they must supply at the agreed price, or they would be at risk of contract breach. That being the case you could buy the same bed elsewhere and seek the difference in price from Dreams, if they were to refuse to honour and supply at the contracted price.