You can setup invoice recipient in Supplier Master and the person attached the supplier as recipient can see the posting proposal lines. However if the person has been set granted as supper access in invoice posting Authorization then he will definitely be able to see the proposal and make changes.

Regards,

Mukesh Gupta

If I get your question right;

there is no direct way of preventing multiple users from working on the same posting proposal.

As explained above, you can restrict unwanted users accessing/authorizing the posting proposal by defining only wanted users as invoice posting authorizers in the basic data set up window.