Allow only one field to edit in Item Master, but no save option


We would like user with specific roles to edit only a particular field in the Item Master.

Hence we have created a custom field and have set the permission level to 2 for that user role. this permission level (2) was given both read and write access.
The default permission level (0) for this user role was set to read only.
I was able to edit the field, but the save button was missing. so not able to save.

If we set the default permission level (0) for this user role to both read and write, i am able to save. But all the fields are becoming editable, which is undesired.

Any suggestion or inputs on how to achieve the above requirement?