Weren't legally obligated, that we know of. There could have been a fee paid at delivery of the deal being made, since that's how this firm and the firm that Musk hired to sue the old firm charge. They keep the billable hours down in order to keep the price low unless they win and then there is normally a "winning fee".

Without context it makes it seem that the payment was out of left field when it was business as usually, especially since Musk didn't want to actually go through with it.