It's just the Sun speculating on a 50:50 possibility. It was well-documented that Matt Smith originally signed a three-year contract which must be due for renewal about now. The Sun's "insight" is basically the same trick played by "psychic" charlatans such as Sally Morgan; presenting bleeding obvious information in a way that seems impressive to the credulous.
It's really a no-lose story for the Sun. If Smith leaves, the "source" was correct and morons will be wowed by the display of insider knowledge. If Smith stays, the Sun can run an outraged story about how the BBC offered him a substantial pay rise. The "big number" will enrage morons who think all BBC staff should be paid the same rate as car park attendants.