I found the following quotation online (on a tee-shirt!), attributed to Libanius:
Men are neither suddenly rich, nor suddenly good.
As an aphorism it is rather like Libanius himself; a bit trite. But did he say it?
I find the saying attributed already in A handbook of proverbs by a certain John Ray, published by Bohn, in 1855, p.451. But of course there is no reference. It seems an uncommon quote, judging from a Google search.
In 1831 a Moral Encyclopaedia, Or, Varlé’s Self-instructor appears, which has the same saying on p.199, attributed to “Laborius” (!).
In 1824 Thomas Fielding’s Select proverbs of all Nations p.207 has it by “Laberius”. This is, presumably, D. Laberius, the Roman knight and writer of mimes who was famously forced to appear on the stage by Julius Caesar in a contest with the actor Publilius Syrus. This seems a more probable source; but how to access his fragments? He is quoted, I believe, by Aulus Gellius; and that is probably the place to start.