Affidavit of financial support - Poverty guidelines

My fiance prior year's income on his tax return doesn't show meeting the poverty guidelines. We're awaiting documents from his employer for the 2023 tax year. Would an employment letter suffice to demonstrate that he now meets the poverty guidelines, or is the tax return transcript necessary for this determination?

Given the time constraints of the 4-month validity period for the approved I-129F petition, we're considering using transcripts from the past three years: 2020, 2021, and 2022. Would this approach be acceptable? Furthermore, in demonstrating meeting the poverty guidelines, does the tax return transcript play the decisive role, or could we present an employment letter from his current employer, indicating that he now meets the requirements? Previously, his tax return did not meet the poverty guidelines.