I would like to know how can I set withholding taxes on Tryton ERP.

The thing is that when I configure normal taxes the amount is added to the base amount as it should be, when taxes are not included in the product price. However, withholding taxes work different as it is deduced from the base amount.

Can you guys help me with some clarification on this?

For example, in Angola, some services incur to withholding taxes. If a total invoice is equal to 100€, and if the percentage of the withholding tax is 10%, the client will pay only 90€ plus the proof of the withholding tax which would be 10€.

This sound like the “Retención IRPF” taxes that we have in Spain. In our case we have a tax with a negative reate (-10% in your case). With this setup the client will only pay the 90€

This is exactly what I was planning to do. I was just not certain if negative tax rates were allowed. So I can see that it’s the right thing to do in this situation. If I configure the tax with negative rate, it will be what I’m look for.

As I said, I started to work with withholding taxes setting a negative rate for it. The problem arises when there are more than one tax, for example IVA, included in the invoice.
For example, if I have an invoice with some product1 with IVA (15%) and some withholding tax (10%), let’s say we have a product1 with price list of 100€:
The total amount of the invoice would be: 100€ (the product price) + 15€ (IVA) = 115€
However, because the product1 has withholding taxes, the invoice will be paid a total of: 115.00€ - 11.50€ (which is calculated with the total invoice amount with taxes included) = 103.50€

So, the way Tryton ERP does the taxes calculation is by summing all the taxes from the product base amount.
For example, following the example above, Tryton ERP would work the total amount of the invoice like this:
100€ (the product price) + 15€ (IVA) - 10€ (withholding taxes calculated with base amount of the product1) = 105€

Really sorry if I haven’t been clear enough. If so, please, feel free to ask any question. I really want to solve this little issue.

Do you guys know of anything on Tryton ERP that could help me with this?

In this case you should set the update_unit_price field of the witholding tax to True and tryton will compute the amount with the previous taxes inclouded. Please be sure to set a proper order (using the sequence field) on the taxes so the withholding tax is applied after the IVA.