iPhone trade-in agreement issue

I am a current Xfinity Mobile customer and wish to trade-in my iPhone XR for a new iPhone 14.  A customer service representative told me I would receive $450 credit for the trade-in, which would be deducted from $629.99 for a new iPhone 14, resulting in a total price of $179.99 or $7.49/month for 24 months.  However, when they sent me a customer agreement form to sign, it said nothing about the trade-in credit or the reduced price; it only showed the full price.  They said that they could not provide anything in writing confirming the credit or discounted price that they offered verbally.  How can they expect me to sign an agreement that is not at all what I agreed to??  I also was asked to type in my full social security number, which I refuse to do for obvious security reasons.

How can I trade-in my phone for a credit and discounted price agreement given to me in writing, and without typing in my social security number?