Ordo Xenos, and the Xenos?

[Scammel]

I'm pretty sure a demon have to abide by the terms of any deal they make.

ATS has a prime example of when a Daemon just says 'stuff it, you're own your own' with no apparent trade-off.

[Iron_Lord]

Even radical daemonhunters rarely make straight bargains with daemons- it's usually a case of them enslaving a daemon- in a daemonhost or a daemon weapon.

Ordo Hereticus I think focus on mutants, rebellions (heresy) and whatnot- not just witches. Some have loyal mutant followers (usually mutated through chemicals, rather than Chaos).

Ordo Xenos vary a lot. Even puritans like Amberley Vail from the Cain books may end up learning Eldar, participating in less-hostile interactions with friendlier aliens like the Tau, or studying rather than destroying alien technology. Similarly, the Deathwatch are not above making use of alien tech.

[daveNYC]

ATS has a prime example of when a Daemon just says 'stuff it, you're own your own' with no apparent trade-off.

Plus any 'deal' would most likely be of the monkey's paw variety. You get what you asked for, not what you wanted.