Translation:– A piece of pie, please. – Savory or sweet?
The question refers to the first sentence which says pala piirakkaa. Because of this relation, also the question needs to respect the grammatical case of the first sentence.
You can test this by adding the adjective into the first sentence: "Pala suolaista piirakkaa" is correct whereas "Pala suolainen piirakkaa" is not.