  In this paper we attempt to understand the role of tin and carbon in magnetic
interactions in Mn$_3$SnC. Mn$_3$SnC exhibits a time dependent magnetic
configuration and a complex magnetic ground state with both ferromagnetic and
antiferromagnetic orders. Such a magnetic state is attributed to presence of
distorted Mn$_6$C octahedra with long and short Mn--Mn bonds. Our studies show
that C deficiency increases the tensile strain on the Mn$_6$C octahedra which
elongates Mn--Mn bonds and strengthens ferromagnetic interactions while Sn
deficiency tends to ease out the strain resulting in shorter as well as longer
Mn--Mn bond distances in comparison with stoichiometric Mn$_3$SnC. Such a
variation strengthens both, ferromagnetic and antiferromagnetic interactions.
Thus the structural strain caused by both Sn and C is responsible for complex
magnetic ground state of Mn$_3$SnC.
