Did you credit cash for $120 and debit debtors for $120?
Sounds like you did the opposite. Am not sure you quite understand debits and credits. You may want to get a refresher as the system logic applies this concept properly.
Little late to be of help, but you can just cancel, and then delete any submitted document in ERPnext. Not a good nor an efficient idea to reset ERPNext every time you make an error.