Carl is right, this is not the proper way to handle this situation.
The HSA should be used only for "qualified medical expenses", not medical expenses that you have reason to expect to be reimbursed for.
What you should do is
1. Pay the dentist immediately with your own credit card (or other after-tax vehicle like a check).
2. When the insurance reimbursement comes in, apply it to your credit card bill.
3. After the insurance comes in, then figure out the difference (the amount that wasn't reimbursed). then call the HSA administrator and ask for a reimbursement of that difference.
4. The HSA administrator will send you a check of pre-tax dollars which you will apply to your credit card bill for the balance of the original dental bill that was paid in post-tax dollars.
You can get distributions from your HSA at any time to repay yourself with pre-tax dollars for qualified medical expenses that you originally paid with post-tax dollars, so long as the original expense was incurred after you started your HSA...yes, even years later.
If the IRS discovers that you have been using your HSA to pay for medical expenses which you know will be reimbursed by insurance (which you then try to put back into your HSA for another deduction), it will look like you are trying to double-dip on deductions. Don't do it.
