Hi,
Return delivery is created against Purchase order and all data is reference to Purchase order. The return delivery and PGI against the outbound delivery becomes part of Purchase order history so it is not possible to change the plant code in outbound delivery which is different then Purchase order.
If you want to return stock back to vendor from plant B then the solution is to create the return Purchase order from plant B to vendor and post 161 movement type material document against this return purchase order.
If original purchase order needs to be reference to create return order then stock first needs to transfer back to plant B and then use the MBRL and PGI ( 122 mov type ) to return the stock to vendor.
Amit.