Surplus is the right word as shown for example here on this Canadian government website.
There they refer to surplus earthwork material which includes more than just soil. I think the potential difficulty with 'surplus soil' is the alliteration but in context I think it will be fine. You may need to differentiate between 'fill' soil that you brought to site and surplus excavated soil that you removed.
Ciria has
Sustainable management of surplus soils and aggregates (RP1124)
but this and the Canadian government's site is more general. As Phil Sweet points out, once you decide you don't want it, excess soil is probably a better word. See the Ontaria govt. site title Handling Excess Soil with
Excess soil is soil that has been dug up, typically during construction activities. It must be moved off-site because it can't or won't be reused at the development site.