Best way to design a Java class that contains an generated ID

I am trying to design an API which involves book a transaction to the system. The system will then generate a transaction ID that uniquely identifies the transaction.

I would like to define the Transaction class as a immutable one, like this:

public class Transaction {
    private final double quantity;
    private final BigDecimal price;

    public Order(double quantity, BigDecimal price) {
        this.quantity = quantity;
        this.price = price;
    }

    public double getQuantity() {
        return quantity;
    }

    public BigDecimal getPrice() {
        return price;
    }
}

The client call the API to create a new transaction:

public void storeTransaction(Transaction t)

But if I do this, how can I store the generated transaction ID?

1. Option 1

I can add an mutable state to the transaction class:

public class Transaction {
    private final double quantity;
    private final BigDecimal price;
    private String transactionID;

    public Order(double quantity, BigDecimal price) {
        this.quantity = quantity;
        this.price = price;
    }

    public double getQuantity() {
        return quantity;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public String getTransactionID() {
        return transactionID;
    }

    public void setTransactionID(String id) {
        transactionID = id;
    }
}

But this will make the Transaction class mutable.

Option 2

I can keep using the immutable Transaction and expose the API like this:

public String storeTransaction(Transaction t)

So instead of save the transaction id in the Transaction class, I could return it to our API user. But this does not look perfect for me as well because the user has to maintain an ID->Transaction relationship.

What is the best practice for designing this type of scenario?