Tithing your income (giving 10% to Charity) – makes you wealthy.
G-d pays back for acts of Charity with material wealth.
How-much-more-so when giving more than 10%:
Up to 20%
Up to everything (as explained in an earlier article)
Giving when there's nothing (via Credit, as explained in the subsequent article)
– G-d will surely pay back with even greater abundance.
Hence the Talmud's particular wording ("Pruta") in this Sign:
"Moshiach will not come until
(Pruta / פרוטה) are depleted."
It is Small Coins only that will phase out of the wallet.
Because when you're wealthy (as you've given so much to Charity) –
you don't deal with cents.
Or with coins all together.
NOTES & REFERENCES
This Sign: "Mashiach won't come until small coins (פרוטה) will no longer be found in wallets." Talmud Sanhedrin 97b.
Giving Charity brings wealth to the Giver: Malachi 3:10, Talmud Taanit 8b.
Note this does not present a contradiction with the earlier interpretations of this Sign, namely (1) people will spend all they have (with nothing retained in their accounts), and (2) people will spend on Credit (because they have no cash): Because all the wealth the Giver is blessed with can and will be poured back to Charity.
Wealthy... don't deal with cents (small coins): In this sense, this Sign of Mashiach does not (necessarily) mean he'll come when all money is exhausted, even coins -- rather he'll come when only coins are exhausted, as opposed to notes and larger-value currency, since people will be wealthy (Rashash commentary on Rashi Ibid).
No more coins... but notes: See Ben Yohoyada commentary on this Talmud section (Sanhedrin 97b).