It wasn't so long ago that selling a law practice was unethical.  So of course, lawyers figured out a way to work around calling it a sale.  Eventually they just amended the ethics rules.  This deal probably conforms to those, but does it violate tax rules?  If the sale was by the S Corp (was it really a corporation, anyway, or maybe an LLC?), the payments should be made to that entity and returns should continue to be filed.  The assets sold may have zero basis, but the S Corp has (you tell us) $345K basis. What are you doing with that?  

The 2024 Form 6252 has a question (at the top, #4) added about whether the selling price can be determined.  Not asked, in 2023.  But there are some regulations going back to 1981, about how to handle these situations.  See the instructions to the 6252.  Google AI is not always reliable but it's a good start:

Case 2: Maximum selling price is not determinable, but the payment period is fixed
If there is no maximum price but the number of years for payment is fixed, the seller recovers their adjusted basis in equal parts over the fixed period.
The seller reports gain in any year that the payments exceed the portion of the basis allocated to that year.If a payment is less than the allocated basis, the unrecovered basis is carried forward to the next year.
If the fixed period ends before the basis is fully recovered, the seller can deduct the remaining basis as a loss in the final year.