When an Org has Multi-Currency enabled, users will see formatting that resembles the following example:
USD 350 (GBP 263.98)
The first value shows the currency that matches the Currency Code of the record, and the second value in parenthesis shows the converted value for the default currency of the Org.
This matches the default behavior seen in Salesforce Orgs with Multi-Currency enabled.
Due to Salesforce limitations, Conga Grid does not support Advanced Currency settings or the disablement of parenthetical values. There is no workaround at this time.