Why does مفلس mean "broke" and not "rich"?
If فلوس are money, how can somebody who is مفلس not have any?
Originally, the verb فَلَس means to "to be empty". The transitive version of this verb is أفلس, i.e. "to lose". From this verb, the noun مُفَلِس comes, noting someone who lost all his money. Also, إفلاس meaning bankruptcy.
As for فلوس (whose singular is فِلْس) it can be a word from outside Arabic and adapted into Arabic for money, just like Dirham (from Greek Drachma) and Dinar (from Latin Dinarius).
On the same line, the scale of the fish is typically called فَلَس (falas). I wouldn't be surprised for wide usage of words with change of vowels or not; This is common in Arabic.