42 USC Sec. 407. Assignment of benefits
(a) In general
The right of any person to any future payment under this subchapter shall not be transferable or assignable, at law or in equity, and none of the moneys paid or payable or rights existing under this subchapter shall be subject to execution, levy, attachment, garnishment, or other legal process, or to the operation of any bankruptcy or insolvency law.
Well, maybe the customer could sign a pledge, but you could not enforce it.
As noted, if SSA funds are co-mingled with other funds as in a DDA, "tracing" is a possibility if you were attempting to exercise a right of offset.
However, as has also already been noted, the fact that the time deposit was previously purchased with SSA payments would be no more relevant than the fact that a car used as collateral was purchased with SSA payments.