The Nigerian striker joined Birmingham on a short-term loan deal in the January transfer window.
Martins has become an instant hit at Birmingham and scored a dramatic winner in the 2-1 Carling Cup final victory over Arsenal on Sunday.
“Martins left Rubin due to personal reasons,” said Rubin coach Kurban Berdyev in the Daily Mail.
“We agreed that he will stay there until the summer and then return to Kazan.
“We haven’t received any formal offers from Birmingham.”