If you don’t have enough money in your HSA to pay for an expense, you can pay for the medical expense out of your own pocket. Once you have enough in your HSA, you can reimburse yourself. If you use your payment card for an amount that is more than the balance in your HSA, the transaction will not go through. If you write a check from your HSA checkbook for an amount exceeding your balance, the check will be returned to you, and you will incur an insufficient funds charge. So keep track of your balance.