Xfinity Target gift card scam

Was called to lower my internet bill 2 days ago.

They offered a Target promotion via gift card. I paid $450 for 18 months. 

They had access to the app and they showed me $0.00 balance. 

Today, they contacted me that there was a problem with my payment and that I  needed to get an additional gift card.

Now I knew that I was scammed. 

I contacted Xfinity. They said they would reimburse me. 

How will this happen? What do I need to do?