All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
What we understood from the question is that you bought the card that has five hundred rupees with a deferred price to six months and you paid its owner four hundred rupees in cash to win one hundred rupees when it is due.
If that is the case, then it is not permissible, because this transaction includes the sale of money for money of the same kind with the deferred date and different amount.
Allah knows best.