There are two ways to do this:
– Make a transaction variant thru SHD0 and assign it to your sales doc. While creating the variant you can place non-changeability ticks on specific fields. Next allow those users only to work with your transaction variant but not with the original transaction.
– You could make use of user-exit FORM USEREXIT_FIELD_MODIFICATION in include MV45AFZZ (via authorization objects, which you can assign in role customizing).
The latter is more flexible but it is not feasible if you want to place restrictions to a large amount of fields.