\documentclass{article}
\usepackage{amsmath} 
\usepackage{xepersian}
\begin{document}
\abovedisplayskip=12pt plus 3pt minus 9pt
\belowdisplayskip=10pt plus 3pt minus 9pt
\noindent
-----------------------------------------------------------------------
$$a=b$$
-----------------------------------------------------------------------
\begin{align}
a=b
\end{align}
-----------------------------------------------------------------------
\begin{equation}
a=b
\end{equation}
-----------------------------------------------------------------------
\end{document}