The Money Bin is a large building that houses a large sum of Scrooge McDuck's fortune, It also serves as an office building for Scrooge.
A 1960s Disney short, Scrooge McDuck and Money, points out that it's financially unsound to keep his millions isolated. It's explained then that it only contains a portion of his money, and the rest continues to circulate. In DuckTales, however, Scrooge seems to keep all of his money there, as evident by him being 'broke' whenever he loses the money inside.
At different points in the series, the color of the bin interior swap between green and blue.
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