How did God provide for the poor saints at Jerusalem after their common kitty ran out?
After their common kitty ran out and they became destitute, God in His sovereignty provided by laying it on the hearts of Gentile believers of Paul's gospel to give offerings for the poor saints at Jerusalem. Their relief came through those Gentile believers' offerings, not through the original shared treasury.
Les explains that the Jerusalem believers had sold their lands and houses and placed everything into a common kitty, so everyone lived on it, and no one lacked (Acts 2 and Acts 3). But even the best investments run out if continually distributed, so after the money was gone they became destitute. Had Israel accepted the kingdom, they would not have needed houses or lands, but Israel rejected the kingdom, and the kingdom did not come in. Yet God in His sovereignty watched over them by laying it upon the hearts of Gentile believers of Paul's gospel to give offerings for the poor saints at Jerusalem.