I am not sure I fully understand, but if the condom slipped off while he was withdrawing his penis (and not during the intercourse), you were not at risk. It sounds like that is what happened.
If you think it slipped off during intercourse, before he was pulling his penis out, then there is a risk and you should test. The 4th generation is very, very accurate at 28 days post-exposure, but if you are more comfortable waiting until 6 weeks, then that is your choice.
The risk is low for a one-time encounter, but you might want to get tested just to be sure.