You can only exclude wages you received in 2020.
Your exclusion period for the 330 days must begin or end in the current year. You could use the period Jan 5, 2020 - Jan 4, 2021 to count your days for the 2020 exclusion period = 362 nonwithstanding any other days in the US.
The wages paid in 2021 cannot be excluded in 2020. A separate test must be applied to see if the wages received in January 2021 can be excluded.
The same period could be used: Jan 5, 2020 - Jan 4, 2021 = 362 current year 4 days you only need a few days for a small bit of of exclusion amount to cover the final pay amount.
Form 2555 Foreign Income Exclusion Instructions
