The landlord, of course, can request the rent you owe. While the "informal notice", whatever it involves, presumably would not satisfy the requirements for the type of notice that would support a court action, as the landlord isn't using it to support a court action that's not an issue.

The DCA recommends, "If the amount of rent demanded [on a 3-day notice] is not correct, it's essential that you discuss this with the landlord or agent immediately, and offer to pay the amount that is actually due. Make this offer orally and in writing, and keep a copy of the written offer. The landlord's notice is not legally effective if it demands more rent than is actually due, or if it includes any charges other than for past-due rent (for example, late charges, unpaid utility charges, dishonored check fees, or interest)." You can use an excessive rent demand as a defense if the landlord later files an unlawful detainer action.