Fizz is trying to scam me

I bought a pixel 8 in late November for a really good price, there was a 349$ discount on the full price and I had a 25% off any phone coupon in my fizz gifts. I paid 517,39$ for the phone. When I received it, the box wasn't sealed. It, had clearly been open already.

I was told by fizz to return the phone under the 15 day return policy, instead of juste sending the phone back and wait to receive a new one. When I told the agent that the phone wasn't on sale anymore, and that I had used my 25% off coupon I was told that I would have to pay the full price of the phone, wait 15 days for the return policy to expire and then ask for my refund. I was assured that I would, in the end pay the same amount as the first phone.

I was refunded 517,39$ for the first phone and I bought a new one for the full price of 1091,11$ I waited for the 15 day return policy to end and I contacted customer service to ask for my refund. They told me they couldn't do it. Best they can do was 50% (which is less than what we agreed upon). They finally accepted to refund me for the full amount. 3 days later I received a 25% refund... I sent a formal complaint and I was told that 25% was the right amount to refund...

I honestly can't comprehend how a company can't understand that what initiated all of this was their mistake and they can't own up to it. Have anyone been in the same situation? I'm really mad about this situation. Fizz still owes me 300$ for a mistake they did and that they refuse to fix.