Welcome to the community @matt86 ! Time to feel 
!! So, HA takes a little bit of time to get your head around, once you have it you will love it like the rest of us! I don’t have a step by step guide, and automating Amber does require an intermediate understanding of the basics otherwise you will get lost. I recommend seeking out some getting started tutorials, examples only below.
However, here is a quick, off the top of my head way to get started with automating restricting your inverter feeding in when price <$0.00
- Install Amber Electric integration (this will answer your question about monitoring Amber’s prices)
- Setup your inverter to receive Modbus calls via TCP (do you have a Fronius)? (Refer first post)
2a. The modbus section needs to be configured including Sunspec model type - Create an automation that will trigger when Amber feedin price is negative - Example is here
Just a hint, this forum is brilliant. You need to ask something specfic and show what you have tried via copy & paste of yaml automation to get the most out of it. Everybodies configuration is different therefore you need to have pseudo logic planned out, at least in your head!
Good luck, we are all here for you!