the original verse should use the character "善", which is from 孟子 盡心上
the verse "窮則獨善其身﹒達則兼濟天下", using the character "濟" is a deviation.
the earliest one i found, is from the letter "與元九書", written by 白樂天, which was in 白氏長慶集 卷四十五.
first of all, it's not talking about charity. then, such claim, implying it's confucianism; is falseness.
intepreting "達" as rich, "濟" as "help the needy", "relieve" is, imo, imaginative, and severely derived from orthodox.
fortunately, the character "兼" is correctly interpreted.
let's examine the version of mr 白.
a manly man (大丈夫) behave (所守) according to the proper way (道), wait (所待) for [his] times (時).
when his times do not come (時之不來也), it's 窮; then (則), he make good (善 ) himself (其身) alone (獨)
when his times comes (時之來也), it's 達; which means he become high official, then (則), his governing accords to the proper way, brings goodness (濟) to all people in the nations (天下)
兼 (both), is used, because, in such state "達", he do goodness to himself, and, to others peoples.
imo, though the character "善" was replaced by "濟", mr 白's usage was similar to mr 孟's, without great difference.
classical text were interpreted, remarked through out history; so, it's better to use these remarks to understand the classical text.
that's culture heritage.
now, my rant, related to the comments:
1 he know the orthodox meaning, and ask for a government post from the communist.
2 he does not know the orthodox meaning, use current usage of character meanings, mislead other by implying it's confucianism.
it's no virtue, or cheating :(
last, a metaphor:
"dog", "green", "hot" & "house" are words taught in kindergarten, everyone know the meaning.
now, a "rich" man claims that "greenhouse" is a house with green color, "hotdog" is a dog that is hot.
how do you response?