Bosch fuel pump costs my GM service

I bought a Bosch Part (fuel pump from Marouns on thursday) and yesterday my Opel Meriva wldnt drive after part being fitted basiclly GM Opel service manager now let me know that the Bosch defective part caused low fuel pressure and cost me 4500.00 because Marouns sold me a defective part! Not at all impressed and if the part isn't refunded in full can safely say that they will of lost a lifetime customer at Midas! And Bosch