I can't pay 924 euro invoice

Hello, 

I have been using NOS Internet for years, recently, like 3 months ago, I got my first mobile from NOS store by paying 441.99 as an initial payment and rest to pay 42 euros every month for 24 months. 

I went to buy another mobile for my wife and since she is not a NOS customer, and I know I have to be a least 2 years of contract with NOS before I quit,  so I already finished my contract term and NOS customer support in NOS store in Oriente, ask me to transfer the service on my wife name and once she has 6 months of paid bills she can have a mobile on installment as well. 

Now, a few days ago I did receive a bill with her name on it which I paid already, but I recieve a bill of 924 euros on my name today :) 

I called support line and explained the situation and they offered me a solution that I can pay 300 and some euros as in advance and pay the rest I think 50 euros every month for 11 months. 
If I knew this is going to happen I would NEVER do that.

now I have few ask, 

1. Why did your people in NOS store put me in trouble by giving this option, they should have informed me before that incase of transfer of service this situation will come up. 
2. NOS should have denied the tranfer when moving service on my wife NIF or should have transferred mobile installments with service on her name as well. 
3. Now I can’t pay that huge amount what should I do ? I am prepared to pay 42 euros every month, if you want I can revert the service back to my NIF.