Subject: Pausing work on [PROJECT] pending payment

Hi [CLIENT NAME],

Per our terms, I'm pausing work on [PROJECT] as of [DATE] because
invoice [#1234] ([$AMOUNT]) is [X] days past due. Nothing is lost —
current progress is safe — and work resumes within [2] business days
of payment.

I'd genuinely like to finish this for you. If there's a problem with
the invoice or timing, tell me and we'll solve it.

Regards,
[YOUR NAME]
