The Uruguayan striker signed a 12-month deal worth £200,000 a week when he arrived at Old Trafford in October after being released by Paris Saint-Germain.
The agreement also included the option of a further 12 months – but only if both parties agreed, says the Mirror.
Cavani was also able to negotiate a kiss-off clause that would see him depart with a £2m golden handshake if United decided against keeping him at the club until the summer of 2022.
The 34-year-old forward can walk away for free at the end of the season – and Argentine giants Boca Juniors want to take him back to South America.
But Boca are unable to match his £10.5m-a-year salary at Old Trafford.
And Cavani can smell a lucrative final pay-day from United that would help cover his loss of earnings on moving to Buenos Aires.