Those who got out will be told they need to give back 50% (assuming you are right that that is how much was left).
I also just looked and a preferential transfer would be when the company that goes bankrupt deliberately pays off some debs but not others, hence giving some creditors preference. The notion does not seem to extend to individuals otherwise regularly conducting business with the company. That can't be resolved the way you suggested. Customers who ran FTX will not be required to return their funds so that someone can distribute some of their money to FTX's creditors, or even so that things can be "fairly" distributed across all customers...
Heck, FTX went out of its way to only allow Bahamian citizens to withdraw money at the end, imagining that would be the jury pool deciding their fate soon.