"We refused to pay the bill and Oracle could not even provide a list of what we'd bought"
"In the end we gave them a list of what we used after performing an internal audit."
That's more like how such negotiations (and it is a negotiation, your money versus their hassle) should be conducted.
If it wasn't a multi $Bn company you'd actually bought software off of it sounds a lot like a version of the "pro forma invoice" scam, not unlike the old "Readers Digest" nonsense.
S/W companies have astonishing data management practices. I've seen very high end development environments where the "contact" listed was the CFO who'd signed the contract.
They had no f**king clue who within IT used it, or who was in charge of IT. All they had kept was the CFO name, which was often years out of date.