On a new proposal anyone with access can set the commission. On an existing proposal the two fields become read-only unless the user has a special edit proposals commission permission. In the current version that permission is not offered on the standard user-role permission screen, so for every ordinary staff role the commission of an already-saved proposal can only be changed by the main administrator account (or a super-admin). You can still change the Sales Agent on a saved proposal — that field isn't gated — but the percentage and fixed amount typed into an existing proposal are silently ignored on save without that permission.