Two bodies are charged by rubbing one against the other. During the process, one becomes positively charged while the other becomes negatively charged. Then

mass of each body changes slightly but the total mass remains the same.

The transfer of electrons from one body to the other results is   development of charges.

Hence, no. of electrons given by one body = No. of electrons obtained by the other.

∴ Mass of negatively charged particle becomes more.

But the total mass remains the same.