the contractor is the LL's choice. After all, it is his property. He has a right to determine who can and cannot perform work on his property.
If you disagree with the billing, then ask the LL to review it with you. as to responsibility: how you stated your situation, it is not possible to determine what understanding there might have been between you and the LL. Apparently the LL saw this as a request from you to have a receptacle installed.
the fact that you had a contractor that you wanted to use leads me to believe you did in fact intend on paying for the installation. If you had no intention of paying, why would you care who did the work? What this appears to be is; you did not get to use your contractor so you did not get the discounted installation you were expecting and as such are now arguing your responsibility for the bill.
Most LL's offer you the residence as is. If you want work to be done (other than legally required items), it is generally going to be on your dime.

