Since your question is lacking context, I'll suggest isolating the statements and log the values. You can start by asking the following questions:

1. What's the value of $result->salePaymentRequestResult->token at that point of code?
2. What happens when you assign a static string to $cart->transaction_id? For example: $cart->transaction_id = 'test_token';
3. Is there any mutator defined for transaction_id attribute? Defining a Mutator
4. Is there any observer defined for the model? Observers