N.J. Stat. § 46:9-7.1
Whenever real estate situate in this State shall be sold and conveyed subject to an existing mortgage or is at the time of any such sale or conveyance subject to an existing mortgage, the purchaser shall not be deemed to have assumed the debt secured by such existing mortgage and the payment thereof by reason of the amount of any such mortgage being deducted from the purchase price or by being taken into consideration in adjusting the purchase price, nor for any other reason, unless the purchaser shall have assumed such mortgage debt and the payment thereof by an express agreement in writing signed by the purchaser or by the purchaser's acceptance of a deed containing a covenant to the effect that the grantee assumes such mortgage debt and the payment thereof.
N.J.S. § 46:9-7.1