How can we restrict an employee entering an expense transaction to not update the corporate rate?

Summary:

We have a scenario where corporate rates are enabled and even after applying tolerance as zero, employee can still edit the rate, and it is not throwing an error. Ideally, we don't want an employee to edit the rates and even if they do, there should be an error due to tolerance which is not happening. Currently, the error tolerance percentage is setup to zero and hence any change to the rate should throw an error but its not happening. Could you please provide a solution?

Note: The environment is redwood enabled 26C.

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