ME22N : Some Item fields should be locked for editing

Dear all,

First of all, I would like to apologise if the solution is already somewhere on this forum ... I have been searching and reading since this morning, but I didn't find anything that could help me to solve my problem ...

My problem is the following: when we open a purchase order via ME22N, we check if there is already a related production order ... If so, some fields on item-level (material, quantity, date, ...) should be locked for editing ...

I tried a lot of user-exits but no one did what I want to do ... There are also a lot of treads about using BADI's, but I am not able to make this work this way ... I probably do something wrong, but I don't know what ...

Can anyone assist me in changing the properties of some fields ?

Greetz,

Kurt.