The right-back’s excellent 20-yard strike capped a training ground corner routine and proved the only one of the game as Unai Emery’s team claimed their third straight home Premier League win over City.
But Cash later explained the ball was not supposed to end up at his feet from Emi Buendia’s pass.
He told the BBC: "It was a nice strike. It's something we worked on yesterday in training, not exactly for me to hit it, it was a special corner but it wasn't meant for me.
"I've had a few like them, I've scored four this season for club and country. On the edge of the box I like shooting, it's what I like to do in training, hit them nice and sweet.
"We work a lot on set-pieces, over the last couple of years there have been a lot of set-pieces that have won us games and today is no different.”
The goal was Cash’s second in the Premier League this season and earned Villa their fourth straight league win, lifting them into the top half of the table.
Villa are the first team since Preston in 1937 to reel off four consecutive victories after failing to win any of their first five league matches.
Cash said: "The last month we've been really good. Defensively really good. We've just stuck together as a group. And at Villa Park they're always behind us."