This is a bit unclear in publication 502.
https://www.irs.gov/pub/irs-pdf/p502.pdf
You can't take a tax deduction for pre-paid future medical expenses. That is explicitly stated in publication 502. I assume that if you pay for an expense in 2024 and the expense occurs in 2025, you can deduct it in 2025, because that's "fair", but that is not explicitly stated in publication 502.
The rules for FSAs are a bit different than publication 502, and the tax regulations place the burden of making sure that the rules are followed on the benefit plan administrator, not the taxpayer, which is why you usually need to submit proof to the FSA plan administrator. However, for HSAs, allowable expenses for HSA are the same as allowable expenses for the deduction, and the burden is on the taxpayer to self-certify that the expenses are allowable.
I think you will be on safer ground if you only reimburse yourself from the HSA in 2024 for expenses incurred in 2024, and reimburse yourself for future expenses in the year they occur. Without doing a lot more research, I can't definitely say that taking full reimbursement now is not allowed, but I am not sure the situation is as the other volunteer suggested. The consequences, if you are audited and the reimbursement is disallowed, is the disallowed portion is subject to income tax plus a 20% penalty.
@dmertz @Hal_Al
