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As such, Tatsugi really should cancel his offer immediately. I'd forgotten initially that GMs can easily spawn otherwise unavailable items, but this also means he almost certainly will be scammed. Many hackers unfortunately know ways to access those GM tools, and - thusly - could easily show him the Sewing Pattern (even if it doesn't naturally exist).
The outfit itself certainly exists - and can even be bought from an NPC easily enough, assuming you have 700k - but there is no known existence of the needed Pattern, since no items purchased from NPCs ever count as "Signed".
It seems Korea eventually received an update that added more Sewing Patterns, which sadly never reached the US afterwards (helping to explain the utter dearth of Patterns available). While I could definitely theorize as to why this might've occured, I'll just stick to the unfortunate facts for now.
As far as I can tell, he's going to - in theory - pay someone just to prove it exists. This is impossible - however - as the Sewing Pattern literally doesn't exist, at least according to various sources. His original post never specifically said the person would hand over the item, just that they needed to prove its existence. It's not like he could actually bring the item back anyways, since he even mentioned using alts to confirm the Pattern's existence on other servers.
@Tatsugi
By the way, I just did some research and - according to Reddit - this item literally doesn't exist on Mabinogi's US servers. I became suspicious when there was no such item listed on our Wiki, and a little more searching quickly turned up the following Reddit post (wherein someone asked which weapon was harder to Erg):
https://www.reddit.com/r/Mabinogi/comments/8tx47u/weekly_questions_megathread_209_62518/ "The most difficult, as as far as I have heard, seems to be knuckles due to the fact that it is currently impossible to pass Erg Rank 6, as it requires a Signed See-Through Cocktail Dress , when the dress doesn't seem to have a sewing pattern available for it." - ADreamtMonochrome
At this point it seems your contest is resolved, but sadly not in a way that helps anyone. As such - rather than offering a cash bounty - you'd be better served to write a bug report, since bringing this to their attention is the only way things might improve. I'm sorry I couldn't be the bearer of better news, but sometimes that's not how things go.