You need to specify the exchange rate only if the currency of the loan is different from the currency of the commitment to which it is being linked.

If the two currencies are different, the mid-rate for the day is taken by default. This rate is displayed in the field and can be changed by you. If the rate entered by you differs from the mid-rate by the normal variance percentage defined for the product, then you are asked for an override. If it differs from the mid-rate by the maximum variance specified for the product, then it does not allow the record to be stored.