Both of those words would mean the vendor was giving you money. Would it not be illegal in your country for a vendor to demand more money from the customer for a product which had already been delivered and paid for?
The correct word is "extortion".
The RM50 is the amount undercharged, if it was a case of undercharge by the vendor.
If I write "He asked for the undercharged", does this sentence imply he was asking me to return the amount to him?
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