If you're after adding horizontal space between columns of a table, this is achieved by defining it along with the tabular column specification:

\documentclass{article} \usepackage{booktabs}% http://ctan.org/pkg/booktabs \usepackage{eurosym}% http://ctan.org/pkg/eurosym \begin{document} \noindent\begin{tabular}{l@{\hspace{4cm}}r} \toprule Detalle & Importe\\ \midrule Adquisición de equipos y software & 1574.17 \euro \\ Material inventariable & 182.45 \euro \\ Mano de obra & 8430.00 \euro TODO \\ Fungibles & TODO \euro \\ \textsc{Total} & \fbox{TODO \euro} \\ \bottomrule \end{tabular} \end{document}
Using @{<stuff>} you can specify what should occupy the space between two columns (or at the left/right ends of the table). I've used @{\hspace{4cm}} to specify a 4cm gap between the columns. Alternatively, you could also have used a column specification lp{4cm}r and added an extra column between the existing two, and not use it to typeset anything. However, then the gap would be slightly larger than 4cm, since it includes some default column separation (\tabcolsep) on either side.
If your motivation for stretching the table is because you want it to fit in the width of the text block, then I'd suggest using the tabularx package. It specifies a column type X, which stretches to whatever width you specify as a mandatory argument to the tabularx environment:
\documentclass{article} \usepackage{booktabs}% http://ctan.org/pkg/booktabs \usepackage{eurosym}% http://ctan.org/pkg/eurosym \usepackage{tabularx}% http://ctan.org/pkg/tabularx \begin{document} \noindent\begin{tabularx}{\textwidth}{l>{\raggedleft\arraybackslash}X} \toprule Detalle & Importe\\ \midrule Adquisición de equipos y software & 1574.17 \euro \\ Material inventariable & 182.45 \euro \\ Mano de obra & 8430.00 \euro TODO \\ Fungibles & TODO \euro \\ \textsc{Total} & \fbox{TODO \euro} \\ \bottomrule \end{tabularx} \end{document}
The use of >{..} is required to push the column entries flush right. This functionality is provided by the array package, already loaded by tabularx.
\hspacebetween tables should work as expected, unless the "second table" wraps to the following line. As such, please add a minimal working example (MWE) that illustrates your problem. This includes specifying a\documentclass.\hlinewill only work withintabularand not between or outside.\\[1cm].