How to link between drafted documents in SAP business one or to edit documents after posting it?

Greetings,

I've this customer who has this business scenario, he has to edit all off the Sales documents after he post it because his customers change their quantities.

So I gave him the 2 standard options : 1st he can save the document as draft, he accepted the idea of the draft, but he asked to link the delivery to the A\R invoice while they are draft.

2nd he may use the return or the credit memo documents, he also refused it because he it will duplicate the work and he will has a lot of credit memos.

So please anyone has a new idea or solution to this case ?!