It has been noticed that if we create an invoice in OpenERP, we see a difference between the subtotal of a line and the base amount for taxes, and this amount can vary hugely as the quantity of the product/item increases. For this, I have been searching a lot and finally the issue can be resolved. Just follow the following steps:

1. Check if the account precision and currency precision are same:

For this, go to the Accounting Module/Configuration/Miscellaneous/Currencies.

Go to the required currency, in my case INR and change the Decimal Accuracy integer.

My account precision is set to 2, so I decided to change my currency precision to 2 as well.

2. Some patch work:

Now in account_invoice.py, go to the compute function of the account_invoice_line model and do the changes.

Replace the line

val['base'] = tax['price_unit'] * line['quantity']

val['base'] = cur_obj.round(cr, uid, cur, tax['price_unit'] * line['quantity'])

Also delete the line

tax['price_unit'] = cur_obj.round(cr, uid, cur, tax['price_unit'])

For reference use the link https://bugs.launchpad.net/openobject-addons/+bug/994445