First, there are a number of ways you could do this in openLCA - get some sort of negative credit for an avoided emission. In this case avoided emissions from landfilling (?) glass. I'll speak mostly to what you have setup already. I can't tell if those are separate providers for that waste glass flow, but I imagine the only thing you would be avoiding are whatever waste treatment exists for that waste glass, now replaced by putting it concrete.
I would have the input "waste glass" as a positive flow. Here you want to connect to any upstream processes that account for the glass actually making it to your concrete facility.
On the output flow, I think you can also keep that as a positive flow and mark it as an avoided product as you've done. openLCA will calculate the impacts of the provider as a negative amount or credit.
I suspect the credit you ultimately get for this will be really small - I don't think glass is going to breakdown and emit anything at the landfill. And landfilling operations are pretty efficiency. Good luck!
