> my client received an email from "abicde@mydomain.com" stating that there is a change in invoice and revised invoice is again sent which had bank account details of a UK bank account.
Emails sent from your domain usually constitute valid contracts. If you're letting other people send emails from your domain because you don't have SPF configured then there's a good chance a court would either rule that you've allowed them to enter into a legally binding contract on your behalf, or else that you were negligent and owe the $10,000 back in damages.
That's why you need to take away the email addresses of people who no longer work for your company, so that they can't enter into contracts on your behalf.
That said no one should ever wire money based on anything they receive via email. So if the sender email had SPF but the recipient just didn't see it flagged because it was in SOFTFAIL mode or whatever, then it's probably the client's fault at that point.