<Message>.send() is a promise, meaning that you can use .then() functions after your message is sent. For example, your code would look something like this:
msg.send({
embed: {
"title": "Success",
"description": `Successfully added ${args[1]}\nUse \`m!stock\` to see it.`,
"color": "#22ba4a"
},
}).then(() => {
msg.send({
files: [free]
});
});
Promises.then()