Coinbase is the exchange service (like Bank) and not the currency, though. It's like having your client pay you through PayPal, for instance.

USDT is the symbol for Tether, one of the cryptocurrencies.

The Currency will be considered as Basis Value on the date of payment, which is whatever he charged them for the work or his agreement to accept this value in lieu of what he charged.

Then, when he converts it to US dollars, he has gain or loss on the currency exchange.