1
along comes a sucker just begging to be parted from his money
See also:
鴨が葱を背負って来る (かもがねぎをしょってくる)
2
double stroke of good luck;perfect timing!;how convenient (for you to show up)!
Other forms
鴨葱 【かもねぎ】 · 鴨ねぎ 【かもねぎ】