What is the ruling regarding a trustee of real estate that the debtor relinquished to the creditor in settlement of his debt, then the debtor reneged on that, and is the trustee obligated to hand over the real estate to the creditor?

What transpired between the plaintiff and the debtor is a compromise for consideration (ṣulḥ muʿāwaḍah), and its ruling is that of a sale in its validity and effects. If the compromise is concluded, neither party has the right to retract it, and the rulings of a sale apply to it. As for the trustee's refusal to hand over the property, this might be at the owner's command, or because the compromise is not documented with him. The plaintiff should refer the matter to the judiciary.