Depending on whether you use ARC. With ARC your example should work, without ARC you need to retain at least the string values.
NSDictionary *dataDict = [foo data];
NSMutableDictionary *currency_dict= [[NSMutableDictionary alloc] initWithDictionary:dataDict[@"currency_data"]];
NSString *time = [data[@"time"] retain];
What anoop-vaidy meant is I think, if you need a mutable dictionary, create that in the data call directly and pass it out. Another note: You can use your knowledge of you data structure in a better way. Instead of building a dictionary with 2 keys, use the first value ( the time string) as the key and the second (the currency dict) as the value. You can access values and keys in dictionaries quite easily
NSArray *value = dict.allValues;
NSArray *keys = dict.allKeys;