The word yield in this case is on par with the English yield as in "give in/give way" not yields as in "yields dividends" so no chance it refers to a clause. I understand the tweet as: Brighton have offered $10m to take Calleri in December however Boca only want to sell in June.
Meaning: If we...
A bit of digging online suggests he's been involved with with the Argentinian TV station Cronica (a major national television news and newspaper) for the past 20 years. So he does sound like a fairly credible source. :shrug: