If they offered to put her in the one you said tou’d Buy from them - what is she in now? Assume she’s a baby from a litter and still with the Mother. We’re they asking much for the cage because if not it could be useful as a play cage when doing a clean of a main cage or even portable enough for a trip to the vet maybe if needed. Anyway - you have a few options I think. If you bought another cage you could take it to the seller for your hamster to be moved into from the start and decline the purchase of the other one (just say you now already have a cage you wish to use.
Or as Soufflé suggested leave her in the little one at first and then upgrade her.
I would be thinking about the money aspect too. The Ferplast duna multi is a good option for a dwarf hamster as well. Much better size than the pets at home plastic cage - not quite as big as the hamster heaven. It’s not easy to find but Portin Aquapet sell it for about £34.
Or you could make a bin cag. Ikeas Samoa bin costs about £15. It partly comes down to whether you want a cage or tank style. A dwarf hammy would be ok with either as long as it’s tall enough to fit a wheel in. Cages tend to have front access as well as top access which some owners prefer (me included
) and just prefer cage styles. However I think the duna multi is the kind of thing you are looking for as you have looked at Perspex cages so far.
However. If they have already moved her into it then Souffke’s suggestion is good. Take the cage and wait a few weeks before upgrading her. There are varying views on cage size for dwarf hamsters- and I agree that the Tosewood Abode is too small for long term use. But for a few weeks until you have something else sorted it could be fine. What people usually do when collecting a hamster is collect them in something like a guinea pig sized pet carrier like the Ferplast Aladino and already have the cage all set up and ready back at home for the hamster to go straight into from the pet carrier and that is then when they need the settling in time. If she is already in the Rosewood cage and only recently been put in it then that is familiar but the change of environment to a new home will still cause stress and need settling time.
I think personally I would worry about the hammy being in such a small cage for a few weeks and want something a bit bigger than that. Is there an option to say no to it now? Or another alternative is put the abode cage inside a bigger one when you get home with the bigger on with substrate in and the odd toy or a house and let her explore out from the abode when she’s ready. Ie leave the to off the Rosewood and keep the top on the new one so just the base of the Risewood is inside the new cage. How are you going to transport her home - in the cage? If so be very careful as ideally they need to be in a pet carrier and can get thrown about in a cage if a car braked suddenly eg.