Many thanks for your patience. It does appear that you have acted reasonably here and that the customer may be trying their luck in getting some of these ‘outstanding’ cost being refunded to them. First of all you cannot be asked to refund the for items which you did not purchase, so if you had paid a specific price for the doors, unless you had purchased the wrong size and should have indeed got the next size up, you would only be expected to refund them the amount you spent on buying them.
If they are keeping the doors instead of having them replaced, then again you would not be expected to refund them for any fitting costs if no such costs have been incurred. You are entirely within your rights to request proof of such costs, together with any proof for decorating work, rather than just having to pay out on what they claim it cost.
The issue is that whatever has happened so far and even if you are entirely in the right, there is nothing stopping them from taking tis to the small claims court. It is their right to take this further in as much as it is your right to challenge that if it happens. They could of course simply be bluffing in the hope that you agree to reimburse them rather than have to face a potential claim so there is no guarantee that they will be going that far, but the possibility does unfortunately exist.
This is your basic legal position. I have more detailed advice for you in terms of the rights you have should they end up making a claim, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you