ICMJE (International Committee of Medical Journal Editors) policy actually has a provision for this specific situation:
If authors request removal or addition of an author after manuscript
submission or publication, journal editors should seek an explanation
and signed statement of agreement for the requested change from all
listed authors and from the author to be removed or added.
It is technically possible to salvage the paper, but the journal is, IMHO, rightfully concerned about the ethics of the process here, as denying authorship on the basis of refusing to pay APC is highly problematic. I would say that the most ethical thing to do in this situation is for you to decide whether you are still okay with paying APC and if so, keep them as authors. That advice is given under an assumption you have already come to an unambiguous understanding of what happened here.
If not, you would first need to get that shared understanding. Can the paper, take a step back and review your work process. More specifically, your point of view can be described as following to the co-authors ("you" collectively refers to the rest of the co-authors here):
- We have all agreed to submit our paper to this journal, AND
- We all knew there would be an APC, AND
- You have confirmed that you are willing to pay your share of APC, AND
- You are now refusing to do so.
If any of these points are challenged, all six (?) of you need to agree on why that happened. Maybe they knew there were APCs, but did not expect them to be that high, in which case it is entirely on them. Maybe you did not tell them clearly enough there will be charges, in which case it is mostly on you. Maybe they have agreed to pay, but circumstances have changed (e.g. experiencing troubles dealing with their institution) - in which case it is on them again, but you might be willing to pay. This is what I meant by "decide whether you are still okay with paying APC" - first of all, figure out if you have enough information about the issue or not.
Either way, most of the scenarios converge at this being a monetary/workplace dispute, which is separate from authorship (see Dirk's answer). Authorship is often reviewed under something collectively known as moral rights:
In continental Europe, moral rights are “inalienable and cannot be transferred or waived.”
In other words, you might have been scammed out of two grand - which you need to figure out and dispute accordingly with them and, possibly, institutions involved. But all that does not make it right to revoke the authorship UNLESS they are now strongly objecting to be associated with the paper at all, in which case you badly need a clarification as well.
The entire situation sounds like a huge breach of work ethics - from your description, they are at fault, but we do not see the other side of the story here (e.g. do you have a clear confirmation, in writing, that they have earlier agreed to paying these charges?).