Material substitution management (1:n)

Question

Dear All,
I need to manage a material substitution 1:n (e.g. material A has to be substituted from B + C + D).
The solutions analysed are:
1) manage a SET material (Z(1PC) = B(1PC) + C(2PC) + D(1PC)) and setup the substitution A(5PC) -> Z = B(5PC) + C(10PC) + D(5PC)
PROBLEM: in this way I cannot manage the partial qty: in case of 100 PC ordered of material A and 80 PC available of material A I should have:
mat A: 80PC
mat B: 20PC
mat C: 40PC
mat D: 20PC
2) try to add new items during the sales order creation/change
PROBLEM: I try to do some test in the user exit and it seems there are so much risk; really it is not possible to do it?
Thanks in advance
Best regards.
Elena.

Answer 1

I would not add new items/materials in userexit.
We used a special program written to split original item (like keep just delivered portion or you can keep qty = confirmed qty), add new item (it can be BOM like your Z material) for the remaining qty. You can copy other info from the original item (if it was changed from default, like texts for example). In this case you can build any logic you want and don't have to maintain these substitution rules.