It's likely to be a rounding error when doing the string->float conversion. You shouldn't use floats when dealing with currency. You could use a NSDecimalNumber
instead.
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
[formatter setNumberStyle:NSNumberFormatterCurrencyStyle];
// Below 2 lines if converting from a "currency" string
NSNumber *modifiedNumber = [formatter numberFromString:modifiedValue]; // To convert from the currency string to a number object
NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithDecimal:[modifiedNumber decimalValue]];
// OR the below line if converting from a non-currency string
NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithString:modifiedValue];
modifiedValue = [formatter stringFromNumber:decimal]; // Convert the new decimal back to a currency string
You may also consider making the number formatter lenient - often helps with user entered data.
[formatter setLenient:YES];