The fact that an account is frozen may or may not have anything to do with OFAC. It could be that the depository bank received a garnishment or tax levy. You won't know unless you a) ask your customer or b) subpoena the other bank. You should be discussing with bank counsel what options you have available to you depending on what the legal proceedings are that impacted the account.
Lesson learned here, if you are going to hold cash as collateral, make sure it is on deposit at your bank.
Sola Gratia, Sola Fides, Sola Scriptura, Solus Christus, Soli Deo Gloria!www.tcaregs.com