Laptop dv4-1242tx
Excess amount of dd

I have purchased the hp laptop and to claim the student offer and i have sent the dd of RS. 5499 and one of the my friend has also purchased the same laptop but he has sent the dd of rs. 3999. and the prize (mobile) which we have got is the same. so i just want to know why is it so... if by mistake i have sent the dd of excessive amount please kindly refund the excess money