Because the cost of DPFs is high due to the precious metals present, it appears that the dealer has provided a new one on an exchange basis, as the old one will be recycled due to its value. Because it has been tampered with this has likely reduced its exchange value, potentially to zero, so although it is disappointing that you have been charged extra as a result, we would expect most garages to do the same if they are providing a new part on an exchange basis.