Does a developer have to dispose of earth excavation?

You can sell this earth!

If they can be sold, the developer definitely will not let them go.

If they can sell it, the developer definitely won't let it go.

Well... even if you get a few euros for the soil... + excavator hours, organization, finding buyers, interference with your own construction schedule... At the end of the day, a wash. So inform the buyer that if he wants it hauled away, it will cost him amount X. Plus Y as proceeds from the soil... then it makes sense.

I would be more worried about how to support the left side... I wouldn't leave the earth wall like that... eventually the sauce will be on the house

You want to have a level surface outside the house. When I look at the picture, that will also be the case once the soil is spread out. Therefore, I hardly believe anything will remain once the soil is nicely distributed. It will turn out well.