As I put on my tinfoil hat...
If he told Monk to "go get his money", or however he phrased it, I'm sure Monk understood that he (Fox) was also looking out for his own interests. I mean, if he had said, "man, I want Monk to stay so we can build something", then that comes across differently. So Monk probably had some thoughts that Fox might not be all in, but having gone through his own struggles, and as crazy as he in on the court, he might be a little more dialed in and realized that Sac is his best opportunity in both the short term and the long term, and so far, that's true.