Amusingly in a thread on email scams and such you didn't read quite closely enough (plus OP didn't do a great job of differentiating either, maybe on purpose) :). Even ignoring the "sent from your domain = contract" assertion:
>Now on the next day 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.
>
>Now an email like "abicde@mydomain.com" doesn't exist at all.
Notice the "i", different from abcde@mydomain.com. He's saying it wasn't sent from the normal email account. The question I'd have is that OP uses "hacked" but there aren't actually any technical details here at all. Was one or the other mail servers genuinely compromised, or someone phished? Or were these emails simply spoofed? Or what? It sounds like it could have just been a forged From which is utterly trivial, every mildly serious spammer let alone spearphisher has done that forever. If the client "asked for a confirmation email" but the "email never reached" because it was a spoofed From and got blackhole'd but the client then took no response as confirmation that would probably be on the client.
Of course whatever the legal case there are other practical considerations, if this is a very valuable client then a certain amount of bending may be in order. It sounds like a pretty hokey order mechanism all around vs even just a simple HTTPS LE plain text web form and static invoice. And there is still the question of how exactly the phishing (if that's what it was) information was gathered for the spoofed invoice in the first place, insider job? Some other leak or hack?
But at least asking the client to try to get the money back seems fair enough. Money in that amount to a developed world bank should absolute be traceable. Alerting the banks and law enforcement should have been the absolutely immediate first move the instant anything amiss was realized. If it was the client's fault and the money really is gone somehow (or even will just take along while to recover) then at least splitting the different shouldn't be unreasonable.