Fix Python function - Accounting OpenERP

The account voucher module is used to apply payments to invoices. We have a multi currency enviroment (MXN and USD). When an invoice get a partial payment it should "Reconcile partial" the invoice with the payment, sometimes it does it sometimes it doesn´t. When it doesen´t the payment appers as it where a Credit note and it starts to get messy and confusing with other payments. I have the case examples when this situation happens, I also know what the solution should be. I need somebody that diggs into the code (150 aprox lines), write a patch to fix it and then do some tests. I will provide a test enviroment with everything setup, also instructions on how to work with OpenERP and the expected result of the function.