1. Click "Quick Sale".>Enter "Customer mobile number" who purchased the prepaid or having prepaid balance.
2. The right side of the customer mobile number has the "Balance " of the customer's wallet.
3. Now select the services which they have taken.
4. Click "Prepaid payment button". 
5. "Save" the bill.
6. If you want to check the balance of the customer's Wallet, then follow step 1.

Note : Prepaid balance can also be send along with bill sms. for this go to settings>Notifications>paste this tag [PREPAID_BALANCE] in sms template.