Alignment: "Breaking out" of environment (enumerate / minipage)

You can put the text in box of fixed size hidden in a box of zero width. Here I have encapsulated this in a \subtotal macro. To make this match you need the total in a corresponding box too. You should calculate the width from the space you know you have left. Note you were missing a \noindent before your minipage.

\documentclass{article}

\usepackage{lipsum}

\newlength{\mytotboxwd}
\setlength{\mytotboxwd}{20pt}
\newcommand{\subtotal}[2][\mytotboxwd]{\hbox to 0pt{\hbox to #1{\hss #2}\hss}}
\newcommand{\total}[2][\mytotboxwd]{\hbox to #1{\hss #2}}

\begin{document}

\setlength{\mytotboxwd}{.1\textwidth}
\noindent
\begin{minipage}{.9\textwidth}
\begin{enumerate}

\item \lipsum[1]
    \begin{enumerate}
    \item Something \dotfill 11
    \item Something \dotfill 84\subtotal{95}
    \end{enumerate}

\item \lipsum[2]
    \begin{enumerate}
    \item Something \dotfill 4
    \item Something \dotfill 300\subtotal{304}
    \end{enumerate}

\end{enumerate}
\end{minipage}

\null\hfill\total{Total: 399}

\end{document}

Do you want something like this? It takes two LaTeX runs to synchronize.

\documentclass{article}
\usepackage{microtype} % not essential, but better typesetting

\usepackage{lipsum,showframe}

\newenvironment{totals}
 {%
  \par
  \addtocounter{envtotals}{1}%
  \settowidth{\totalswidth}{\ref{\theenvtotals label}\quad}%
  \setcounter{totals}{0}%
  \begin{list}{}{\leftmargin=0pt\rightmargin=\totalswidth}\item\relax
 }
 {\par
  \addtocounter{totals}{-1}\refstepcounter{totals}\label{\theenvtotals label}%
  \noindent\hfill Total:\subtotalbox{\ref{\theenvtotals label}}%
  \end{list}
 }
\newcommand{\subtotalbox}[1]{\makebox[0pt][l]{\makebox[\totalswidth][r]{#1}}}
\newenvironment{quantities}
  {\begin{enumerate}\setcounter{subtotals}{0}}
  {\unskip\subtotalbox{\thesubtotals}\addtocounter{totals}{\value{subtotals}}\end{enumerate}}
\newcommand{\quantity}[2]{%
  \item #1\dotfill#2%
  \addtocounter{subtotals}{#2}%
}
\newcounter{envtotals}
\newcounter{totals}
\newcounter{subtotals}
\newlength{\totalswidth}

\begin{document}

\begin{totals}
\begin{enumerate}

\item \lipsum[1]
    \begin{quantities}
    \quantity{Something}{11}
    \quantity{Something}{84}
    \end{quantities}

\item \lipsum[2]
    \begin{quantities}
    \quantity{Something}{4}
    \quantity{Something}{300}
    \end{quantities}
\end{enumerate}
\end{totals}

\end{document}