If your client issued the payment in 2023, they may consider it a 2023 expense and put it on your 2023 form 1099-NEC. However, if you don't receive the payment until 2024, you would (generally) consider it as 2024 income.
The easiest thing to do is report whatever the 1099 says. Assuming your income is more or less similar year over year, this won't change your tax significantly in 2023 or 2024. However, you would be permitted to ignore that income for 2023 (even though you get a 1099) and instead report the income in 2024 (even though it won't be on a 1099 for 2024). If you want to do that, and aren't sure how to make the adjustment, write back here after you get the 1099 and are preparing your return.
