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Rates for rate-based expense codes, such as mileage, are typically not editable. For these types of expenses, your administrator will generally specify a set rate (for example, they might specify that mileage expenses are paid at $0.80/mile).
For these types of expenses, you are only required to enter the quantity (for example, the number of miles traveled), and the expense amount is calculated automatically.
If you can't see a Quantity column, click the Settings button at the bottom, and enable it.
You can also enable the Rate field to view the rate your administrator has set for that expense type.

If desired, administrators can allow users to override rate-based expenses.
Refer Solution How to allow users to Override Rate-Based Expenses?