Default Dimensions -In General Ledger>Main Accounts>Legal Entity Overrides>Default Dimensions - the user can define a fixed or no-fixed value. When the transaction(journal entry) is created using a fixed value - the user can change the fixed value - however when the voucher posts - the fixed value is entered on the voucher.

Default Currency- In Main accounts - when a user defines a default currency - the validate currency code option -Required allows a user to change the required currency however when the voucher posts - the required value is entered on the voucher.

Option #1 Idea: User should get a notification before posting to indicate the value will be changed upon posting.

or

Option #2 Idea: Fixed Value defined should not be allowed to be change on the line.