Thank you for your response. But the problem is that some suppliers uses 4 decimals and they apply a rounding per line,other suppliers apply a rounding to the base. It depends on the software used by the supplier.

I am thinking that the solution can be introduce a "virtual" product in the supplier invoice, which price is the difference for adjusting the base.

I think it is be a very common problem because it depends on the way each supplier makes its invoices. I would like to know this issue is usually solved in OpenERP?