Foster, a salaried handler at the East Lothian yard for the past four years, had previously been assistant to Brian Meehan, John Gosden and Peter Chapple-Hyam.
He said: "The decision was taken due to the current financial climate. We've had 31 winners in my spell here which isn't too bad.
"It's very disappointing and unfortunate as the horses have been running well but we've been struggling financially for quite some time. I'd love the opportunity to train again in the future. I still have interest in three horses including Angelo Poliziano, who has won twice on the all-weather this winter, and they are all going to Doncaster sales next week."