If you can't return the leather then it's time to try to clean it up. Oxalic acid as Tom suggested. It's used in the States to clean cement floors, driveways, areas around a pool. Paint stores are a good source here.
I've never tried it but I have heard that lemon juice also works. You'll need to dilute it since you have so much area to cover.
Once clean, just experiment with the finishes you have available until you find one that works. A finish will be necessary or the leather will become stained quickly.
Dan