FinCen 105 form, although normally handed to CBP, goes to Treasury. You can download and fill it out before arriving at the port of exit.

With a declaration there's little CBP can do to "hold" the money. If you owe any government any tax, that would have nothing to do with CBP, and is not yet due to be filed--whether federal income tax ("next" year) or state sales tax ("next" month or quarter). The FinCen 105 provides a record for Treasury (and IRS) at a later time.

A FinCen 105 in hand will also defuse any suspicion at TSA--as you should have filed the form before TSA (assuming at port of exit).