What do you mean "there is no law?" Of course there is a law. There are many laws. We are a nation of laws and Trump, much as he want to, cannot make up his own rules. 

Now, as to DOJ grants, the department cannot make up its own new requirements for grants and funding. It cannot attach new conditions. Congress, not the administration, makes the laws and Congress set the requirements. If Denver meets them, it qualifies. Sessions can't add to to them.

So please cite what section of US Code defines "sanctuary city" and show us how Denver s violating it. As far as I have seen, Denver is abiding by every single immigration law. The problem here is that you and Trump and other anti-immigrant people want to make up new rules and try to force cities and counties to do the feds' job. Ain't gonna work.