At one point in their history, the Travellers met Abraham Derris-Cuthbertson. They made a deal with him wherein he would provide spacecraft for them to leave their world, and he would mine resources from it.
The Travellers eventually returned home to find their planet full of toxic waste, about to be made a dumping ground. Despite the urgings of the Seventh Doctor and Ace, they refused to leave the planet, trusting that Derris-Cuthbertson would honour his contract, which he finally did. (PROSE: Last Rites)