If I understand correctly, you have entered into a binding purchase agreement to acquire an REO property from a lender, and you had intended to assign your interest in that agreement to the end buyer for a small premium/profit, but this buyer wants to give you the money to buy the property, secure the money with a deed of trust, and then have you "sell" the property to him a for a little more than what you had purchased it for. This is doable, although it's not clear why the end buyer wants you to do it this way. Of course, you are taking the risk of acquiring the property with the end buyer as your lender and then having that buyer/lender decide not to purchase the property, in which case you are stuck paying a mortgage on the property until you find a another. Assuming everything goes as planned, the second escrow will entail a transfer of the property to the end buyer, thereby extinguishing the deed of trust by way of merger. Upon transfer of title, the mortgage is paid off and you receive from the buyer an amount of money equal to your profit on the transaction.
You need to pay an attorney to assist you. Deeds of Trust secure loans. Deed of Trust with Assignment of Rents just provides the lender with full security in the event you default on your loan obligation.
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I agree that you need an attorney to assist you in these transactions. The transactions could easily be messed us without the proper documentation.
I am not your lawyer, and I am giving you this information to help you find a lawyer. Do not rely on this information. Consult a lawyer.