Although that control may be disabled when you haven't activated Multicurrency within the company, it could also be disabled if the GL offset account selected is limited to home currency. I think there is a behavioral difference when you perform bank transfers as well, so there might be some data entry scenario where you select a type, enter and save, then edit the entry and a expose a problem. It might be a change to an account that allows multiple currencies, and change to a home currency account or change to a different bank account. I'm also curious if it was a pre-defined type or a custom type you created in Customize CM Transactions (Set Up CM Options / Transactions) that was used and the effect configured for the type.

Verify if you are using Multicurrency within the company (try to open any MC task if you don't have access to SM). If you identify the transaction type used in the original entry, check the bank account configuration in Set Up Bank Accounts, as well as bank accounts used in any transfers. Review the offset accounts in Maintain GL Accounts, and verify the Effect on Cash for any custom types used in Set Up CM Options. There might be a bug in the code but it could be some action or setting that is exposing it. If you know the exact entry sequence that creates the problem, post the steps to duplicate.

If the transaction(s) is posted, the best way to balance these is to create reversing and/or balancing entries. Reversing entries are going to have the opposite effect on your bank and GL accounts with an offset amount followed by an entry that would have had the original effect. These would have to be reconciled manually in your bank reconciliation.