My client deferred 1/2 of SE tax in 2020 under the CARES act. The amount was $1,068. He paid half of that by 12/31/2022 pursuant to the payment schedule. When I filed his taxes for 2021, I did not opt out of the loss carryback (lesson learned) and now he has received this notice from the IRS stating that he owes $534 and interest. Is this the money that he would have owed by 12/31/2022? Are they charging interest because the loss carryback made him ineligible to defer his SE tax? I got him to fill out a POA and I have been trying to call the practitioners line every day and I cannot get through.