Magento 2 – How to Get Selected Payment Method on ‘sales_order_save_after’ Event

I've had a similar problem (removing dynamically payment methods) and solved it by extending the _canUseMethod($method) function in Mage_Checkout_Block_Onepage_Payment_Methods. There you can return false for specific payment methods which should not be shown in the payment selection.

Example:

class Company_Module_Block_Methods extends Mage_Checkout_Block_Onepage_Payment_Methods {
    protected function _canUseMethod($method) {
        if ($method != null && $method == 'checkmo') {
            $carriers = Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress()->getShippingMethod();
            if ($carriers == 'flatrate_flatrate') {
                return parent::_canUseMethod($method);
            } else {
                return false;
            }
        }

        return parent::_canUseMethod($method);
    }
}

The config XML would look like:

<config>
    <global>
        <blocks>
            <checkout>
                <rewrite>
                    <onepage_payment_methods>Company_Module_Block_Methods</onepage_payment_methods>
                </rewrite>
            </checkout>
        </blocks>
    </global>
</config>

That way I could remove payment methods right before they get shown in the one page checkout.

Hope that helps anyone.

You can try with the sales_quote_payment_import_data_before.
This one is dispatched in Mage_Sales_Model_Quote_Payment::importData that is called by Mage_Checkout_Model_Type_Onepage::savePayment that is called by Mage_Checkout_OnepageController::savePaymentAction, which is the action called when you press continue on the checkout after selecting the payment method.