Could you point to the exact description of this requirement. For me, it is a very strange requirement because the principle of immutability of posted accounting is very common and general.
More over there is the post_date and post_number that have the constraint/design to follow each other order.
There is already Remove check_invoice_sequences (#5205) · Issues · Tryton / Tryton · GitLab
Why are the standard reports not good?
Which one is missing? We have AEAT111, AEAT115, AEAT303, AEAT347 and AEAT349.
For me, it is really a feature to please people who can not adapt. It will not provide any functionality except introduce complex code and bugs.
This is a feature that for me is not possible to implement right. It was discussed on Add warning when an supplier with same reference already exists (#8244) · Issues · Tryton / Tryton · GitLab
Discount display is a marketing thing so I think it is better to implement it on sale.
We had an old prototype but it will be better to start from Discount on Sale