console.log

#<ActiveMerchant::Billing::PaypalExpressResponse:0x007fd3a2667178 @params = {
    "timestamp" = > "2012-02-18T15:00:36Z", "ack" = > "Success", "correlation_id" = > "fd42c44f29d95", "version" = > "62.0", "build" = > "2571254", "token" = > "EC-08B34690L3027751N", "transaction_id" = > "1Y616105WY000492X", "parent_transaction_id" = > nil, "receipt_id" = > nil, "transaction_type" = > "express-checkout", "payment_type" = > "instant", "payment_date" = > "2012-02-18T15:00:34Z", "gross_amount" = > "750.00", "gross_amount_currency_id" = > "USD", "fee_amount" = > "22.05", "fee_amount_currency_id" = > "USD", "tax_amount" = > "0.00", "tax_amount_currency_id" = > "USD", "exchange_rate" = > nil, "payment_status" = > "Completed", "pending_reason" = > "none", "reason_code" = > "none", "protection_eligibility" = > "Eligible", "seller_details" = > nil, "success_page_redirect_requested" = > "false", "Token" = > "EC-08B34690L3027751N", "PaymentInfo" = > {
        "TransactionID" = > "1Y616105WY000492X", "ParentTransactionID" = > nil, "ReceiptID" = > nil, "TransactionType" = > "express-checkout", "PaymentType" = > "instant", "PaymentDate" = > "2012-02-18T15:00:34Z", "GrossAmount" = > "750.00", "FeeAmount" = > "22.05", "TaxAmount" = > "0.00", "ExchangeRate" = > nil, "PaymentStatus" = > "Completed", "PendingReason" = > "none", "ReasonCode" = > "none", "ProtectionEligibility" = > "Eligible", "SellerDetails" = > nil
    }, "SuccessPageRedirectRequested" = > "false"
}, @message = "Success", @success = true, @test = true, @authorization = "1Y616105WY000492X", @fraud_review = false, @avs_result = {
    "code" = > nil, "message" = > nil, "street_match" = > nil, "postal_match" = > nil
}, @cvv_result = {
    "code" = > nil, "message" = > nil
}

Probaste:

response.params["TransactionID"] ?