Default Sales part impact on Labor line invoice for a Work Task

Step 01

Add a default sales part to a Resource Group

Now add the above resource group as a demand to a work task and manually add a different sales part from the default sales part defined above. Do the time reporting and authorization 
 

Now check the planning line and observe the manually entered sales part is successfully shown as expected.

But when Sales Tab is checked manually entered sales part is replaced by the default sales part defined under Resource Details.

Is this the expected behavior? In that case it is not possible to manually enter sales part and do invoicing when default sale part is defined against a resource