Purchase amendment doesn't work with new lines

I have the following problem with purchase_amendment…

initial purchase order made with 6 articles, and an agent went to supplier with the PO to take quick delivery … where it became apparent he needed to manually write in an additional article to the order.

The delivery note contains this additional article.

Amending the PO in Tryton, we try to add a ‘change line’

But the ‘line’ field is mandatory - and if I select ‘search…’ then click on ‘new’ to add a line, all the fields are greyed out… impossible to save because ‘quantity’ is required for a line!

How to add the new write-in item?

NB deletions are simply setting the quantity to zero.

It is not supported on purpose: Amendment for Sale and Purchase

Then the term amendment should not be used. https://en.wikipedia.org/wiki/Amendment

Please read correctly the definition before making annoying and false statement.

Amendments can add, remove, or update parts of these agreements

This definition includes without any problem the current behavior of the module.

In Amendment for Sale and Purchase you mention that you don’t want to manage certain things in the amendment.
But to ‘create’ the missing ‘line’ first, in order to change the quantity, why is it impossible to not simply invoke the form for purchase.lines.new() forcing quantity=0 in order to do such? It already displays such a form but with everything greyed out.

By the way, the amendment also seems to omit updating the delivery date.

can does not mean must.

no, can means able to or capable of
at present, purchase_amendment CAN NOT add

We need to find a way to fix (or amend;-) that shortcoming.