I took a quick look at the Mishna Berura 108.
He gives examples of when not to daven a makeup Shmoneh Esrei, from which it seems pretty clear that in the case of one forgetting it by Shachris, not davening Mussaf, and then remembering it by Mincha time, he can say an extra Shemona Esrei then.
If one davened Mussaf, there is no makeup.