Medical expenses you pay in 2024 can only be deducted on your 2024 tax return. (And, because of how the deduction is limited, you may get no actual benefit, but that depends on your other tax situations.) You can't shift the expense to another year. And, if you expect to get reimbursed tax-free in 2025, you should not report the deduction at all since that would be double-dipping (getting a deduction and a tax-free reimbursement).
