Hi and welcome Charlotte,
If your customer withdrew but the condom was still inside you, I would say the chances of infection or any sperm escaping are very slim. It was only the condom he slipped out of, not you. Did he ejaculate inside the condom and was the stuff still in it?
For peace of mind and only if you want to, if you're not on any contraceptive, take the morning after pill and also make an appointment with the GUM clinic. If you can do a walk-in instead of waiting for an appointment, even better. But bring a good book because it can be a mind-numbing wait.