Ask HN: Pending/failed card transactions for the exact balance of my account

I'm in the UK and have a regular payment to my orthodontist for $385, which works out as about £290 at the moment.

Yesterday my balance was £170.68 and I got a pending transaction to my ortho for £170.68 which meant I couldn't spend anything. (I don't have an arranged overdraft).

My ortho couldn't see this transaction on their end.

Later on, my ortho tried the usual amount (~£290) and it got declined.

I can now see another pending payment to them for £245.80, which was also my exact available balance at the time.

I've spoken to the fraud center and the person insisted this was just some kind of coincidence. I called my bank, and the person I spoke to there admitted that this was highly unlikely, but also didn't have any explanation for it. He said that in order to do this, the company requesting the payment would have to know my balance.

The fact that they are requesting dollar amounts makes it even weirder, but apparently at my bank the £245.80 shows as a request for something like $325.29, and I think he said the £170.68 one was a request in pounds, although this could have been a miscommunication between us.

What could be causing this? Is there some component within the card system that's seeing these transactions and min()ing them with my currently available balance? That seems unlikely but also the only possible explanation here.

The card is a Visa debit card with Co-Operative Bank.