how to pass denomination details to api??

Check this:

public class DenominationCalculator
{
    public static string CreateDenominationString(List<(string Denomination, int Count)> denominations)
    {
        List originalDenominations = new List
        {
            "DN2000", "DN1000", "DN500", "DN200",
            "DN100", "DN50", "DN20", "DN10",
            "DN5", "DN2", "DN1", "DC20",
            "DC10", "DC5", "DC2", "DC1"
        };
        StringBuilder denominationString = new StringBuilder();

        foreach (string denomination in originalDenominations)
        {
            bool exists = denominations.Any(d => d.Item1 == denomination);

            if (exists)
                denominationString.Append($"{denomination}#{denominations.FirstOrDefault(d => d.Denomination == denomination).Count}~");
            else
                denominationString.Append($"{denomination}#~");
        }

        return denominationString.ToString().TrimEnd('~');
    }
}

class Program
{
    static void Main()
    {
        List<(string Denomination, int Count)> denominations = new List<(string, int)>
        {
            ("DN2000", 2),
            ("DN1000", 1),
            ("DN5", 2)
        };

        string denominationString = DenominationCalculator.CreateDenominationString(denominations);
        Console.WriteLine(denominationString);
    }
}

yeah but how can i do that in code behind and as u said how should i compare my actual entered amount with this type of logic?

DN: Means a bank note.
DC: Means a coin.
#: Separates the denomination from the quantity.
~: Separates different denominations.

For example, in the string "DN2000#~DN500#4~DN200#5~DN100#112~DN50#~DN20#~DN10#~DN5#~DN2#~DN1#~DC10#~DC5#~DC2#~DC1#~DC20#"

DN2000#: This means there are denomination note of 2000 currency units, and there's no quantity. Because after # there is nothing specified.
DN500#4: This means there are 4 denomination note of 500 currency units.
DN200#5: This means there are 5 denomination note of 200 currency units.
DN100#112: This means there are 112 denomination note of 100 currency units.
DN50#, DN20#, DN10#, DN5#, DN2#, DN1#: Similar to above, with no quantity.
DC10#, DC5#, DC2#, DC1#, DC20#: Similar to above, but for coins instead of notes.