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The word has been seen in the manga ハヤテのごとく！
After finishing last, Hayate is given a card with no money on it but “unlimited debt”:
The full sentence reads
そちらは入金が一切ない代わりに十五{とご}で無制限に借金が出来る『うしじまカード』となっております。
What could 十五 mean here? If it does mean “fifteen”(percent?), why is it not read じゅうご?
As @Jimmy Yang's linked page suggests, トゴ is slang used in the context of usury, meaning 十日{とおか}で五割{ごわり}. Taking the first kana of each word we get トゴ. It is an illegal practice to loan money at an abusive interest rate: 50% every 10 days. In Japan it is known as 闇金{やみきん} (black market loans). Such high interest is called 高金利{こうきんり}.
トゴは10日経過するごとに5割の利息がつく方式で、3つの中では最も高い金利です。トゴで10万円を借りると10日で5万円、20日で10万円、30日で15万円の利息です。(source)
10日で5万円、20日で10万円、30日で15万円 しかし闇金は複利なので、10日で5万(借金が15万になる)、20日で5+7.5(←15万×0.5)=12.5万(借金が22.5万になる)、30日で12.5+11.25(←22.5×0.5)=23.75万の利息(借金が33.75万になる)です😨