The easiest way would be to set these rights on an entity level. In the domain model, open the ProposalDetails entity, go to the “Access rules” tab, and make sure you give admin / approver write rights, whereas the user should only have read rights on this particular attribute.
If you need globally for this attribute , you can go with the Eline Bijrek suggestion or if you need only in the particular page you can create two text box with the visiblity condition based on the roles , one is for admin with editable and another one for user with non editable