Customer specified by customer_id does not exist in Braintree_ClientTokenGateway->_verifyGatewayResponse()

This is likely due to syncing of databases, switching Braintree accounts, or purging the Sandbox. In essence the site
believes that a user should be in the vault, but it cannot find them. Best solution for this is detailed below.

It happens because we run generate $token = Braintree_ClientToken::generate($arguments); , which could fail if it has already been passed a customerId which (for one reason or another) is not valid. Therefore a check beforehand would be ideal to ensure the ID exists before using it. If it does not exist, then we reset on the Drupal end, and move on.

diff --git a/web/sites/all/modules/contrib/commerce_braintree/modules/commerce_braintree_dropin/commerce_braintree_dropin.module b/web/sites/all/modules/contrib/commerce_braintree/modules/commerce_braintree_dropin/commerce_braintree_dropin.module
index d54b75c..bfbaf53 100644
--- a/web/sites/all/modules/contrib/commerce_braintree/modules/commerce_braintree_dropin/commerce_braintree_dropin.module
+++ b/web/sites/all/modules/contrib/commerce_braintree/modules/commerce_braintree_dropin/commerce_braintree_dropin.module
@@ -253,6 +253,23 @@ function commerce_braintree_dropin_submit_form_elements($payment_method, $order)
 
   // Initialize Braintree and create a token.
   commerce_braintree_initialize($payment_method);
+
+  // If this token cannot be found in the Braintree system AND Braintree has
+  // initialized successfully, then we need to strip it from the database (as
+  // we have liked switched Braintree accounts, or purged Sandbox data)
+  try {
+    // Try to find the user.  if this fails, we get an Exception back
+    $token_test = Braintree_Customer::find($arguments['customerId']);
+  } catch (Exception $e) {
+    if (!empty($user)) {
+      // Unset the variables, and remove permanently from this user (as we will
+      // now be saving the new customerId instead)
+      unset($arguments['customerId']);
+      unset($user->data['braintree_vault']);
+      user_save($user);
+    }
+  }
+
   $token = Braintree_ClientToken::generate($arguments);
 
   // The custom token is required to generate the Drop-in payment form.

Above is the code I tried, I will attach a patch to a comment in a second.