I apologise if my language is a little unclear on this question, I've changed a few details to make the situation more generic.
I'm a Software Widget Expert for department A at a medium/large organisation. It might be relevant to mention my department is known as a bit of a 'maverick', and has a reputation for going against the usual processes of other departments. One online widget I manage relies on a subscription to an external software. Now this external software is inexpensive (around £30 a year, nothing for a large company) and functions by use of a special key file that acts as a licence. A few weeks ago, I made an error and overwrote that file, I assumed it wouldn't be a big issue and reported it immediately in the hopes that we would have an appropriate backup. For reasons that I don't think are relevant to the question and due to the nature of the external software the vendor is unable to refund/give us a new key.
Unfortunately, because I'd overwritten the file the same day as it was created, the backup didn't cover the period where it existed. This is somewhat of a problem. Purchases for this external software are managed by department B, so I contacted them via the usual channels to raise a new purchase.
This is where a problem begins. Department B rejected the purchase because while our online widget is not functional, it is indicating an error unrelated to the external software. Therefore, according to them that's not the problem. I explained the reason for the misleading error, and the reasons for needing a new purchase of the external software. They refused again, with the backup of their department superior. My own superior has agreed to back me in my request, but so far we are at an impasse.
How do I protect myself in the resulting fallout?