(Required) A valid transaction ID from a previous purchase, such as a credit card charge using the DoDirectPayment API, or a valid billing agreement ID. Billing agreements are valid until the agreement is canceled by the buyer; whereas, transaction IDs are valid for 730 days from the transaction date.
You must have the buyer's agreement to obtain funds from the buyer's account. The buyer must log into PayPal once to agree, but after that, PayPal does not require the buyer to log in. This agreement represents a billing agreement between you and the buyer. PayPal maintains the agreement, and the buyer can cancel the agreement by logging in to PayPal.