Keep partner copied from reference doc. during new partner redetermination

Hello,

I´m trying to sort out if it is possible to keep a particular partner number (forwarding agent SP) copied from a reference document when sold-to is changed in the new document.

I will explain myself, currently my client creates sales quotations and inside they fill the forwarding agent partner function(SP). Later they create a scheduling agreement taking this quotation as a reference and all partners are copied to this new document. Until here all works fine. The problem comes later, because they change the sold-to and partners are redetermined deleting forwarding agent. They want to keep this forwarding agent copied from quotation reference document.

I have tried to use user exit EXIT_SAPLV09A_003, but it doesn´t work: there is no way to get previous partner numbers, only we can get new sold-to we have just filled so it´s useless.

I tried as well marking this partner function, SP as not modifiable in partner function procedure, but this doesn´t works, although it appears in grey after changing sold-to it is deleted automatically.

I have searched if there is any BADI I can use but apparently it isn't any executed during partners redetermination.

Can anybody help me?

Thanks in advance and best regards,

Raquel