We do not wish to use Sales Part Cross reference functionality .

@klorimer, yes in the later updates Sales Part Cross Reference is imposed in several flows. I have come across few in Incoming CO flows. What is the context of your flow?
In one situation, we opted to use GTINs only in the incoming message, where it helped to function without using the Sales Part Cross Reference.  

@Vimukthi Mahakumbura thank you for your fast response.   This is a bit of a pain that for every sales part we will need to have a cross reference per customer ?  Why would this not have remained optional?  We do not require both Customer Part No. and a Sales Part No. in the incoming message as both are the same.  

@Vimukthi Mahakumbura sorry I  should have stated that it is in the Incoming Customer Order we are getting the error.

@klorimer, yes I agree with you, it can be cumbersome. But that is required basic data. What I hear from RnD is that, there have been several developments in this area, and that has caused this change. 
May I know what information is passed towards your ICO, with regards to the part no information? 

Thank you @Oshada Samarasinghe we will try and use your option 1 , leave the customer part number null. We just need to understand which field to update for the Sales Part No. and not the Customer Part No. 

The below is the current setup Customer Part No. , unsure of which line needs updated for the Sales Part No.