Check the boxes outside the smithy. The green slime slab should appear on one of them... it's a bit small though so look carefully.
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Yeah, tried that. Nothing works.
I finished the whole slime dragon event and the inn was upgraded. Before I did anything else, I talked to the wizard, get the black thong from him, used it in the room, gave the slime suit to the wizard, and finally i exited the inn for the first time. Talked to Jaler and Kuno before I went to the smithy, but found nothing about the slime slab.