If the receipt total/amount field is using the Payement Txn field - "Transaction Amount", then this will show the amount with the Currency field. For example as shown below

To changed this, select use the Letter Form Builder [Beta] button on the Letter object and drag/drop another field - for example "Total Amount" to replace this field.  You can change the column heading to still have the label "Amount" if that is required.

The updated receipt will now NOT have the Currency, but will show the Currency symbol.