Besides losing face, Kham was sure that the pay offered for the run would now be less than it was. With Darke's personal interest, that price would have been Kham's going rate. Calling Grabber back, making himself look hungry, would drive the fee down. If he took the run at the lower price, word of it would get around and that would also be bad for business. Once a shadowrunner's price starting going down, it wasn't likely to go up again. The jobs would get cheaper and cheaper and eventually you'd face a dirty run for dirt and then you'd end up under the dirt. Kham wasn't ready for that, so he let the troll go on walking.