TC 640: Advance Payment of Determined Deficiency

What the code actually does

IRS Document 6209, Section 8A (the IRS’s reference guide to its data-processing codes) titles TC 640 "Advance Payment of Determined Deficiency or Underreporter Proposal" and defines it as crediting "the tax module with an advance payment of a determined deficiency" — the module being the account record for one tax period. It is money paid toward tax that has not yet posted as an assessment — a deposit against an expected TC 300 or TC 290.

Practitioners informally call these deposits "cash bonds," and Doc 6209 specifically flags the 990-999 blocking series (the ninth through eleventh digits of the document locator number) for that use, referencing Revenue Procedure 2005-18 for the governing rules on deposits made to stop interest from running on a disputed liability.

The rule that surprises many clients: no interest on a cash bond's excess

Doc 6209 says: "Overpayment interest is never allowed on TC 640 (blocked 990-999) 'Cash Bonds' which are in excess of the audit deficiency to which it is applied." It adds that "overpayment interest is not allowed even if the deficiency is subsequently abated in whole or in part." The IRM narrows that: the excess over the assessed deficiency comes back without interest, but where the bond was applied to the deficiency as a payment of tax at assessment and the deficiency is later abated, interest runs on the abated amount from the assessment date (IRM 20.2.4.9.1). A client who overpaid a cash bond deposit gets the excess back with no interest on the time the IRS held it. A returned IRC 6603 deposit also posts as TC 640 and can earn interest, to the extent the deposit is attributable to a disputable tax (IRM 20.2.4.9.2.1).

Rev. Proc. 2005-18 replaced the cash bond procedure for remittances made on or after March 28, 2005. The interest difference is concrete: under IRC §6603(d), the statute treats a deposit returned under the Rev. Proc. 2005-18 mechanism as a payment of tax for purposes of §6611 overpayment interest, to the extent it covers a "disputable" item — so it can draw interest on the way back, at the federal short-term rate (IRC §6603(d)(4)) rather than the higher standard overpayment rate. For a new remittance the choice is between a section 6603 deposit and a payment of tax, except that section 6603 does not apply to employment taxes (IRM 20.2.4.9.2): the IRS generally treats a remittance without the written deposit designation as a payment (Rev. Proc. 2005-18, sec. 4.01(2), with exceptions in secs. 4.04(1) and 4.05(3)), and a new remittance cannot be a cash bond (IRM 20.2.4.9.2.1), except that an employer in a worker-classification examination who has not yet received a notice of determination under IRC §7436 can make a cash bond deposit to stop interest from accruing and still preserve the right to go to Tax Court (IRM 20.2.4.9.2). The client must send the written designation with the remittance, naming the type of tax, the tax year(s) and the disputable tax (Rev. Proc. 2005-18 secs. 4.01(1), 7), not discover it afterward.

The freeze the deposit creates

Doc 6209 also documents a double-sided freeze: the module is "frozen from refunding, offsetting out or credit elect when TC 640 posted and no TC 300 present," and separately "frozen from offsetting in" until a TC 300 with an earlier 23C date (the IRS's formal assessment date) exists. In plain terms, the deposit sits inert — unavailable to the taxpayer and unavailable to absorb other balances — until the audit actually closes.

The freeze releases automatically once the module reverses the TC 640s, a qualifying TC 300 (or TC 29X in certain blocking series) posts, or the module balance reaches zero or debit. Until then, a client asking "why can't my cash bond be applied to my other balance" has a structural answer, not a service failure.

What people mistake TC 640 for

People confuse it with an ordinary subsequent payment, which is available immediately for offset and refund. The module freezes a TC 640 deposit until a release event posts, such as a qualifying TC 300, and interest depends on whether it is a cash bond or a 6603 deposit.

People confuse it with the eventual assessment itself. TC 640 is a payment, not a liability determination; the actual deficiency posts later, typically as a TC 300 examination assessment, against which the deposit is then applied.

Its dishonored-check reversal, TC 641, reads like an audit reversal. It is not — a bounced cash-bond check reverses the deposit and, absent a secondary TC 280, generates a TC 286 bad-check penalty (governing authority discussed on TC 641's own page), unrelated to the underlying audit's merits.

What to do next

Before sending money, decide whether to designate the remittance in writing as a Rev. Proc. 2005-18 deposit. An undesignated remittance generally posts as a payment (Rev. Proc. 2005-18 sec. 4.01(2)).

Do not promise a client interest on an anticipated refund of an excess cash bond; Doc 6209 says overpayment interest is never allowed on a cash bond's excess over the deficiency.

Confirm the freeze conditions before telling a client the IRS can redirect the deposit to another balance — it usually cannot, until a qualifying TC 300 posts.

Track the corresponding audit or underreporter case to its TC 300/29X conclusion, since that is what actually releases and applies the deposit.