If the mobile app invoices are not getting the correct invoice number from BUSY,you can maintain seperate invoice series,To do this configuration,you can Follow given steps:
Step 1:Go to Administration menu,
Step 2:Click Configuration,
Step 3:Click Voucher series Configuration,
Step 4:Click on sales,
Step 5:Add new series for mobile app,
Step 6:Click voucher numbering and set numbering automatic to this series,
Step 6:After that Go to the Administration and click users,
Step 7:Then click cloud sync users,
Step 8:Add cloud sync user,create name and password,
Step 9:You will find an option here is a transaction rights, and you will see a checkbox for sales and receipt.Check that box,and there you can tag the series you created for your mobile app.
Step 10:Set local and central sale type as itemwise.
Step 11:After done,all those configuration, your voucher numbering will be properly maintained.