"he tells me that he also had "a line" of meth. When he was taken in he signed a paper stating where the meth came from so that he would not be charged with meth. Could the EXCEPT be them going back on their word? after him sigining the paper."
Yes, that's exactly what it is.
Thinks like "Tell us where the meth came from and we won't charge you with possession of it" is just the sort of thing that cops say to suspects to get them to incriminate themselves.
The cops offer not to charge him with the possesion is meaningless. Cops don't decide who to charge with what, the DA does. Cops are allowed to lie to suspects, it's a standard part of any interogation.