You have a bill now then you could pay it now ... then as the money is put into the HSA each pay period you take the same as a distribution to reimburse yourself for the amount you paid out earlier.
You can take the distributions per pay period, per month or all at the end of the year depending on your situation. Since medical bills usually don't incur interest you could just take a monthly distribution from the HSA and make the payment in installments.
