I am still confused. The client did have taxable Social Security Benefits but that is already in the equation to determine Adjusted Gross Income. Also, the IRS website states how the 3rd payment is to be calculated and based on their document, the client received all he was due with the EFT payment that was received in March. It appears the IRS is making mistakes reconciling these payments.

Footnote:

I just spoke with the same client today (7/24) he just received a letter from the IRS on his 2020 return indicating he was due an additional refund because the Recovery Rebate Credit was incorrect. He did not receive the full $1200.00 on payment 1 because his adjusted income in 2019 was higher. He only received $887.80.  When preparing the Rebate Recovery Worksheet it allowed him an additional credit of $312.20 for a total of $1,200.00 on the first payment. The second payment of $600.00 was received as well. 

In the IRS calculations they are saying the Recovery rebate credit should be $801.65 not $312.00.