Damage math can be tricky. You are out of pocket only $100,000.00 but your claim was really for $400,000.00. So, perhaps the damage amount of $400,000.00 could be contested, if your parents forgave you the balance or otherwise released you from the debt. However, I think that the claim is legitimately for the full amount because this is what you lost, even though you had borrowed it in the first place.
The bigger problem is proving attorney malpractice. You have to prove that you would have won the underlying case and that the attorney did something wrong to cause you to lose it. That’s a pretty tough burden to meet in any circumstances.
Best of luck to you.
This response is intended to be a general statement of law, should not be relied upon as legal advice and does not create an attorney/client relationship.