deducted the without any information by vodafone

Location/place: bihar , patna

Name of company/service: vodafone

i am using my Rs.4 pack of 25 MB given by company in mobile no 8051947447. i am busying in download music . at my last download i verifies my remaing MB it about 6 to 7 MB left and i continues my downloading and after somemintues its stop and i see my balance it is Rs.0.39. i call in coustmercare but it repies that your MB is over so your Rs.30 is deducted.if my MB is over so they have to show or give message so they donot work propely or its their plan to deduct balance and gain marginefrom me and us .