How do I calm my father down when he feels he has legitimate concerns?

Get a BIG stamp that says PAID IN FULL...
Stamp all receipts "PAID IN FULL" and show those to him.
Some people need that visual. And older people are more apt to "believe" something that says "paid" rather than look at a receipt that looks pretty much the same as the bill with little lettering that says...auto payment received. Or full payment will be deducted on...
(by the way hide the stamp so he can not find it.)

My mom obsessed over deadlines and bills and gas in the car, etc. I created a white board calendar and put the bills and paid beside them. She kept it in her bathroom so she could see it. Since I had the bills on autopay, it was easy to go in and update the paid date. After a while, it stopped bothering her but she didn’t want me to take the board. When I put the same info on paper, she lost the paper and we looked for it. The white board was better for her.

Perhaps you could get a large "Paid" stamp and stamp the bills as they are paid and show that to him when he questions you about it?

My 90-year-old mom who has dementia is very focused on her estate accounts and bills. Every month I review all her finances with her, but she does not remember. Her new memory is almost gone. Sometimes she asks me about her accounts on a daily basis. What I do every month is put her monthly invoices marked "paid", and her estate accounts statements in a clipboard. Every time she asks. I tell her to look at the clipboard and sometimes I show her the paid bills. It gives her comfort. Of course, now she is forgetting about the clip board and where she puts it. So, I hide it and when she asks, I tell her where it is. It seems to give her comfort. I also do not worry if I have not time to change the statements every month because she can't tell which month it is about 50% of the time. I hope that you find a solution and I wish the best.

I came here to say the exact thing that @Moxies just wrote:

If you simply can't get dad to understand that the bill is already paid, sit down with him, and write a check for the amount as he watches (then secretly void the check). Address envelope, put voided check in, pretend to stamp it, and walk out to mailbox to pretend to mail it.

It's a long shot but maybe this charade will ease his mind about that one bill on which he is fixated. Good luck.

You have to learn how to ignore with love. Show him the proof of the bill being paid a couple of times then let that be the end of it. Do not take any more questions about it. Do not show the proof again. Ignore the repeating and do not allow any further discussion about it.
Your father's behavior is common for people with dementia. You must ignore the topic of a dementia loop. It is the only way to possibly break a person out of one. You answer a couple of times and then that's it.
Try distracting him with something else. It may work, it may not.
You'll just have to ignore with love. It's the only way.

Why not try CBD Gummies. My husband has gotten great relief from delusions regarding imaginary things/tool that he has left behind and jobs he has to get finished. We can tell when the dose is wearing off in about 4 hours and his anxiety begins to return. It takes about 15 min. for the comforting effect and then he returns to his usual confused but cheerful self.

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Often dementia or a confused / changing brain doesn't respond to logic.

You need to focus on his emotions and feelings (I know you are wanting / thinking you are doing this by showing him PAID invoices).

Perhaps along the lines:

I know you are very responsible paying your bills.
You are paying your bills and I will always assist you in doing so.
I appreciate how concerned you are.
It is important to acknowledge (reflective listening) his words back to him:
Yes, your concern of paying your bills is very legitimate.

Calming him might only happen when you change the subject to shift his mind / fears to something calming.

Try any approach that 'sticks' or works. And, any approach may change with each passing day.

He might 'just' need something else to occupy more of his time, i.e., tv, a book, a puzzle, knitting (?) painting, going in the garden or to a park. Fill up his time with different experiences. See if this helps.

Many older people get on a repetitive roll of comments.
Acknowledging their concern and moving on (distractions) seems to be the best most of us can do.

Gena / Touch Matters

Hi Carrie, I went through something similar with my own dad during his last few months. He always paid his bills on time but sometimes he was anxious. He would never allow autopay- he was very much old school- so what I ended up doing was calling whatever company he paid and letting their automatic system read off the balance, which was always zero since he paid by check well within the payment window. That always made him feel less anxious. Maybe you could do the same thing and put the call on speakerphone so dad can hear it with you. If you do this, I suggest that you call ahead first on your own just to make sure the payment has posted to his account. Good luck to you.

CarrieP: Sometimes an elder will be unable to handle financials. My late mother used her town's bookkeeper to balance (NOT) her checkbook. The bookkeeper was off by $659 FOR TEN MONTHS. My mother stated "She'll get it right on month number eleven." My response - "No, she is finished; she doesn't get eleven chances; she either balances or amends the financials on the first month" That was absurd.

You can mark the bill paid IN BLACK MARKER for your father.