documentclass{article}

usepackage[T1]{fontenc}
usepackage{lmodern}
usepackage[utf8]{inputenc}

usepackage{marginnote} % Enhanced marginpar
usepackage{todonotes}

%
% We need to redefine marginpar.
% marginpar does not work inside floats, but marginnote does.
% Also todo uses marginpar internally and this way it actually uses marginnote.
% See manual for marginnote and sec. 1.6.8 in the manual for todonotes.
%
renewcommand{marginpar}{marginnote}

begin{document}

begin{figure}[htbp]centering
begin{minipage}{.5linewidth}
This is a sentence with a footnote.footnote{It only serves as an example.}
Now a sentence with todo{The footnote ends up in the margin.}a texttt{todo}-note follows.
However, the last footnote is printed at the end of the minipagefootnote{Because it is inserted after the texttt{todo}-note.}.
end{minipage}
end{figure}

end{document}