You would report a SEP contribution $600 less than originally deposited. The $600 of contribution returned is treated as never having been contributed.
Note that if the SEP contribution fro 2023 was made in 2024 and you expect to be able to contribute at least $600 to a SEP IRA for 2024, you could just leave the $600 in and treat that $600 as part of the SEP contribution for 2024. It's your responsibility to track the year for which particular contributions are made. The SEP IRA custodian only tracks the year in which a SEP contribution is made, not the year for which a SEP contribution is made. Of course if the $600 contribution was made in 2023 for 2023, you'll have to remove it since it can't be treated as a contribution for 2024 if it was contributed before 2024.
