Set field A to readonly when field B is changed ( field B is changed by a workflow)

1. field A ( field in header) and "Revenue"(two option set) is on the same form of opportunity entity.

2. field  "Revenue"  is updated by a workflow.

3. I wanna set A to readonly when  "Revenue" field's value is changed to "system calculate" by workflow.

Q:

1. i try to add a business rule to do this but it not work.

2. i try to do this in js when form is [reload] or "Revenue"  is [on change] , but when workflow is done it seems neither [reload] nor [on change] is called.

Any Ideas?

Thank you