Jase...

You only have one option mate which is to cooperate with the dibble and give them the unit and the receipt,if you give it back to the mush now the dibble could charge you with conspiracy as you are in effect conspiring with him to get shut of the evidence(he will probably just dispose of it)and then you are proper fooked.....

He stiffed you with it,let him sort it out with the plod.
I know none of this gets your money back but it could save you a prison sentence and the stigma of a criminal record....