scum up

When someone acquires new and often expensive gifts via an acquaintance, always acquired without the acquaintances knowledge.
Can happen with or without the owner of the item being present however. At times however a person committing a scum up will even remain at the house with the victim and help them look for their new item in an effort to look like the true friend the unsuspecting victim still thinks they may be

A tactic often employed by a someone who is jonesing

Like a come up but scummier

See also: scummin on thru'