Why am i not able to change in my voucher such as quantity and amount etc when trying to modify the invoice?

If you are not able to do the changes in your voucher such as quantity and amount etc follow these steps to get this resolved:
Step 1: If you have logged in with subuser in that case you need to login with superuser 
Step 2: Then click on administration menu and then click on users submenu
Step 3: Open that user master from modify or list in which you are unable to do the changes in voucher
Step 4: After that make sure you have selected allow price change in voucher option as Y to enable it
Step 5: Then check the menu rights option available at the top right of user master and click on transactions menu and click on voucher such as sale,purchase etc
Step 6: After that expand it and check you have all rights of there such as modify,add or view after that save the usermaster and then again by login with subuser and check by modifying the voucher details.
Step 7: If still facing the same issue in that case you need to go to the administration menu and then click on configuration submenu
Step 8: After that click on voucher series configuration
Step 9: Then click on voucher such as sale,purchase etc and then select the series name and click on voucher configuration
Step 10: After that click on configure advanced POS options
Step 11: Then check te amount,quantity and price field in the bottom left
Step 12: If it is selected as Fixed convert it to variable and save the changes and then check by modifying the amount from the voucher.