鴨が葱を背負って来る

[kamo-ga-negi-wo-shottekuru]

Literal translation:

A duck comes with a leek on its back.

Meaning:

A very favorable thing

Situation in which this idiom can be used:

If naïve sightseers enter a rip-off shop, the shop owner can use this idiom behind their backs.

鴨 or カモ [kamo]: a (wild) duck