Just put land 'for sale'.... and keep lowering price until it gets snatched up. If you price it below L$1 per sqm it ought to sell fairly quickly (a week maybe - which can be a lifetime in SL!!)
I never did like how auctions worked out anyhow. I believe you can 'request' to buy abandoned land at L$1 per sqm via a Support Ticket. But to get rid of it, quickly, abandoning is only choice.