1st, wasn't Japan on the highway to decades-long economic stagnation at the time? 2nd, selling WWII spoils is probably a sure way to sink public support instantly for any Russian leader, let alone Yeltsin.
It might be possible if Russia gets a lot worse economically to resort to selling clay, but it's still a very long shot.
The islands have Oil and Gas reserves and these reserves where not discovered at the time, plus the Russians could do with instant capital at the time.
If there's no Oil and Gas discovered why the heck would Japan want it?
Historically, even when Japan possessed half the island prior to WWII, the government notoriously failed at incentivising enough Japanese people to move there.
Why would it be any different in 1991?
During the time if japanese rule the island was a large provider of coal.