Yes, you must enter a 2024 date since the sale occurred in 2021. You can simply add an investment sale for the percentage of the payment that is capital gain in 2024 with no cost basis and a 'Various' purchase date. Of course you will need to enter 2024 as the sales date, however you can name the investment sale '2021 Installment Sale Payment' to identify it.
Add the interest income as if you did receive a 1099-INT. This will take care of the sale for the trust return for 2024. The name of the payer will be the buyer.
Note: Since all depreciation recapture had to occur in the year of sale, the only taxable gain now is capital gain that would end up on Schedule D anyway.
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