Hospital Now Demanding Nearly Double The Amount After Bill Already Paid In Full Weeks Prior

Hello all. My wife and I have been living in Japan for 5+ years now, and she works at a US Navy base here. Due to our visa we are ineligible for NHI, unfortunately. We'd happily pay out of pocket, but it's prohibited for us. We love living here, but this is a new one for us.

We've both been going to a local municipal hospital here when needed, and they've always charged us 150% burden vs what a local on NHI would pay which is 30% of 100%.

Wife has been going to them to monitor some neck lumps for 2 years, and recently they decided to remove 2 of them to biopsy and test for cancer due to rapid growth. We paid the bill in full like usual, and at the follow-up a week later they said there was a mistake on the bill and the new amount was nearly double the original amount that we already paid. I don't know if they are changing the burden or changing the services rendered because the woman did not offer any details other than the amount nearly doubling. FWIW the follow-up had the usual 150% burden, so I'm thinking they are either doubling the burden to 300% or changing something else, but only for this specific bill. This is by far the most expensive bill we've ever had here between the both of us since moving here, so maybe they're getting greedy for just this one bill? I'm not sure. We will find out in 2 weeks for the results of the biopsy when they will refund the previous payment and issue a new bill for the previous date. This is already a problem with our US insurance who has already processed the original bill claim.

This has never happened before, but with all the hate for foreigners going on here lately it feels a lot like they are squeezing us out of pure greed. I hope I'm wrong and somebody legitimately made a mistake, but if they're just upping the burden for no reason that will be very disappointing. It's not like they're already providing some additional benefit for paying 5x more than a local pays let alone 10x which is what this new bill would represent if they're only upping the burden to increase their profits. We have to bring our own translator as it is.

My question is if hospitals are allowed to decide to modify a previously paid bill and nearly double the price after a bill has already been paid in full at time of service? First post here and looking for insight please.