Fix sign of buy command (display only)

client-barcode/freitagslib/commands.py

@@ -26,7 +26,7 @@ class DepositCommand(Command):
 class BuyCommand(Command):
 	def __init__(self, item):
 		self.item = item
-		self._difference = item.price
+		self._difference = -item.price
 
 	def run(self, user_name):
 		net.buy_item(self.item.id, user_name)