Is it permissible to conceal one's real salary from one's mother and lie about its value, for the purpose of accumulating money to use in times of need?

Kindness to one's mother is among the greatest acts of drawing closer to Allah, while lying to her is丑陋 and forbidden. If the questioner is compelled to conceal some of her money, she can use equivocations instead of lying, with the caveat that it is not permissible to waive rights through equivocations.