Dynamic List Reference

I am creating a multiselect dynamic list and would like to include specific information from the records in the picklist. For example:

Deposit: $300
Payment #1: $500
Payment #2: $600

The payment amount would come from the id reference. I am able to do this using the following code:

Reservations = Rentals [ID == input.Reservation];
    paymentlist = List:String();
    if (Rentals[ID == input.Reservation].Deposit_Balance != 0)
    {
        paymentlist.add("Deposit: $" + Reservations.Deposit_Balance);

What I am struggling with is how to then reference this item and update the record when this item is selected? I have tried this code (added on update of the picklist field), but it does not seem to recognize that the item "Deposit: $300.0" has been selected.

if (input.Payment1.contains("Deposit: $" + Reservations.Deposit_Balance))
{
    show Deposit_Due;
    disable Deposit_Due;
    input.Deposit_Due = Rentals[ID = input.Reservation].Deposit_Balance;
    show Deposit_Paid;

Is this possible?