How did Bod escape from the pawn shop in The Graveyard Book by Neil Gaiman?

Poor Bod has been locked in a back room by a greedy pawn-shop owner called Abanazar Bolger. Bod has in his possession a valuable brooch. Bolger knows just how valuable this piece of jewelry is, but isn't prepared to pay Bod a fair price for it; he'd much rather steal it from him. He'd also dearly love to know where Bod found the brooch so he can get his grasping hands on more treasure.
Bod hoped to pawn the brooch to buy Liza Hempstock a gravestone. When she was alive, Liza was partially drowned and then burned at the stake as a witch. Although she really was a witch, Bod takes pity on her; he thinks it rather unfair that Liza doesn't have a gravestone of her own, so he steals the brooch from the Sleer, and for the first time, ventures outside the confines of the graveyard to see if he can raise some money for the young witch spirit.
Meanwhile, back in Bolgar's pawn-shop, Bod tries to figure out a way to escape. He can move through walls in the graveyard alright, but in the world outside the graveyard, it's impossible. Thankfully, Liza's on hand to help Bod out of a jam. Immensely grateful at Bod's risking his neck to buy her a gravestone, she says a few magic words, and Bod is able to fade away, leaving Bolger and his accomplice scratching their heads when they unlock the back room door.