I found a 30 riyal recharge card and sold it for 20 riyals, so what is the ruling?

A recharge card is considered a 'found item'. If it has value, it must be announced. If it has no value, then the one who finds it may benefit from it and own it. As for selling a recharge card for less or more than its worth, there is no objection to it because it is the sale of a utility.