Business income for which year?

Just want to get confirmation on this.

 

For my service in 2020, I will be receiving a payment in 2021 (before March 15) from a Trust, as the payment is based on a % of the prior year's profitability.  The Trust is treating this as an expense for 2020.

 

However, I'm using the cash method of accounting.  I'm wondering this is a 2021 income for me or 2020 income? What's the catch if any?