I think the sale may have been reported incorrectly in 2023. If it's not a publicly traded stock, then there were two options.
1) report the entire sales in 2023 including the future sales proceeds (excluding interest) and you are done.
the future proceeds are reported in the year received but with a cost = to the proceeds so no gain or loss. or
2) use the installment sale method of reporting using form 6252. You report the total sales price and the tax basis. However, you are only taxed on a portion of the gain using the ratio of proceeds received to total proceeds received and to be received.
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for 2024, under option 2, you would enter the proceeds and interest received on the 6252
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for publicly traded stocks only option 1 is available
interest is taxable in the year received and you probably should received a 1099-INT
