Han levde av hans arv under de tre åren före Azkaban, eftersom han var upptagen med att slåss med Orden.
Sirius farbror Alphard lämnade honom mycket pengar mot slutet av sin vistelse på Hogwarts. Det var nog för att han skulle köpa ett hus, och senare en Firebolt, där de återstående fonderna fortfarande bidrog till en betydande andel av Potters förmögenhet.
“Where did you go?” asked Harry, staring at him.
“Your dad’s place,” said Sirius. “Your grandparents were really good about it; they sort of adopted me as a second son. Yeah, I camped out at your dad’s during the school holidays, and then when I was seventeen I got a place of my own, my Uncle Alphard had left me a decent bit of gold — he’s been wiped off here too, that’s probably why — anyway, after that I looked after myself. I was always welcome at Mr. and Mrs. Potter’s for Sunday lunch, though.” Harry Potter and the Order of the Phoenix - Chapter 6
Omedelbart efter Hogwarts gick Sirius till Dumbledores fientliga Order of the Phoenix för att kämpa mot Voldemort och skulle inte ha haft tid för några stora jobb under de tre åren innan han gick till Azkaban.
By the time the four friends left school, Lord Voldemort’s ascendancy was almost complete. True resistance to him was concentrated in the underground organisation called the Order of the Phoenix, which all four young men joined.
Pottermore - Remus Lupin