field VBAP-WERKS grey and cannot be changed in return sales order

Hello,

I have created a sales flow to be delivered from plant A.

I have created a return flow and goods should be received a plant B.

when I create my return order, the plant remains being grey with plant A and I cannot change it.

when I click on the match code for this field, I can see my two plants A and B, but I cannot choose B.

Can you please tell me how make sure to have the choice between both A and B plants in the return orders?

Thanks in advance for your precious help,

Kind regards,

K.