In this paper we prove a topological lemma on real valued random variables which implies the basic ingredients for the proof of the Fundamental Theorem of Asset Pricing in the two period case. In particular, previous results of Stricker and of Schachermayer are special cases of our result. Our proof is considerably shorter and more transparent than previous proofs of related special cases. / Series: Working Papers SFB "Adaptive Information Systems and Modelling in Economics and Management Science"