Can I exchange leftover Sri Lankan rupees at the airport.

Crucially, you absolutely must present the original exchange receipt from when you first bought the Rupees. The banks need this receipt to prove you legally brought foreign currency into the country before they will convert it back.

The exchange rates for buying back foreign currency at the airport are usually quite poor. It is much smarter to carefully manage your cash and spend your remaining LKR on souvenirs or food before heading to the airport.