Retaining Tax

How can I setup FA so that tax is retained, both in the customer invoice, and the supplier invoice?
I have read the wiki and searched the forum extensively

In Colombia, depending on a company's tax responsibility:

1. It is necessary for a customer to discount my invoice to them by 3.5% as an initial tax payment which is paid monthly to the Tax Administrator
2. It is necessary for me to retain 3.5% of a suppliers invoice as an initial tax payment which is paid monthly to the Tax Administrator

How can I do this in FA and show this in the GL?

What I have tried.

Allowing negative tax rates would achieve this, is this possible?
Using the customer / supplier discount functions would also achieve this, but prevent me from using these functions for their correct purpose.
From what I understand the quick entries function would do this for the supplier, but I do not know how to set this up for the above settings? Can someone please show me?
Also from my understanding of quick entries, they cannot be used for customer invoices?

Any help would be appreciated