The number to confirm early is the current balance, because a reverse mortgage compounds semi-annually and grows every year it is outstanding, so the payout figure your lender quotes needs to be current to the actual possession date, not an old statement.
Where the balance has grown close to the home's value, the remaining equity after payout may be smaller than expected, and that is worth knowing before you list rather than after an offer is accepted, since it changes what a fast sale actually nets you.
A quick payout request to the lender and an accurate value for the home are the two pieces that need to line up. Get your hand-reviewed valuation from Joel Dyck.