Correspondence related to the paper by Busemann and Feller, Zur Differentiation der Lebesgueschen Integrale [On the differentiation of Lebesgue’s integrals]. Fundamenta Mathematicae 22 (1934) 226–256.

Herbert Busemann (1905--1994). A biography for his Selected Works edition

“…His plans to emigrate are reflected in his correspondence with Courant; cf. [6] and [7]. Courant expressed some will to help Busemann and came up with various suggestions.…”