Solving for Principal Balance (X). when we know the principal and interest payment, term and rate

Matt_6360  ....  Better answers come quicker when you provide a concrete example, ideally in an Excel file that you attach or upload to a file-sharing website.

Presumably, you are talking about a debt ("unpaid balance").  But it is unclear if it is a loan (type=0) or a lease (type=1), for example.  And there are many more details that you omit.

Suppose the original "normally-amortized" loan is $100,000, to be repaid with 180 equal and regularly-occurring payments (e.g. monthly) at 1% per payment period, and you want to know the remaining principal (balance) after 90 payments.

You might estimate that with the following formula:

=FV(1%, 90, payment, -100000)

The "payment" (combined principal and interest; not one or the other) might be calculated by:

=ROUND(PMT(1%, 180, -100000), 2)

Alternatively, "payment" might be any arbitrary amount.  When you wrote ``we know the principal and interest payment``, that is probably what you mean.  I misread that as "payments" (plural), as if the interest payment were a fixed amount, which would not be a "normally-amortized" loan.  Mea culpa!