Probably one of the most requested service, Airtime to Mpesa reversal has been quite a pain in the ass, particularly since this is a thing that is done by individuals who badly need the money. Within the Mpesa USSD menu, Buy Airtime sits right below Withdraw cash. This poses a very easy possibility that one might miss the touch controlled menu entry and go the wrong way.
The results have for the longest time been disastrous with no official solutions. This would cause individuals to purchase airtime with money they depended on for basic needs like food or pocket money. Enterprising individuals here and there have come up with solutions which the Mpesa host Safaricom has not been pleased with, ending up with blocking of the said service that rode on a pay-bill number.
At last Safaricom has opened up to the possibility of reversing the transaction if you contact them via social media and make them aware of your situation. They have a disclaimer though, if you spend even a cent of the purchased airtime you are on your own.