HSA- Health Savings Accounts

We currently offer HSA's and are considering adding a debit card option to access the account. Right now the accounts are a simple savings where the member can make transfers within their own account. I'm looking for any policy guidance in the HSA area. Our biggest concern in offering a debit card option is if the account goes negative what do we do: transfer from another account, deny the transaction etc.

Thanks

sissyl@northshorefcu.org