Credit limit customer differs from credit limit customer in header

My user has tried to change payer in sales order line item Ex: A to B and got an error message "Credit limit customer differs from credit limit customer in the header "

Now the user reverted payer to original one ''A" and there is a checkbox in the partner as "Item Partner"

If the user tries to change partner in the order header now from "A" to "B" system gives an error message Credit limit customer differs from credit limit customer in the header " Why is this a standard behavior?

My Question was

1) I rejected the 1 line item which is having checkbox "item partner" as there are other line items in sales order and tried changing in the order header partner payer from "A" to 'B' gives an error Credit limit customer differs from credit limit customer in the header

Is this a Bug?