For the "Strictly Business" quest, capture Susan Lancaster and receive your reward for her capture. Afterwards, speak with her in the slave quarters part of Paradise Falls. Note: You will need one of the slaver's keys to access the slaves quarters; pickpocket one. After entering the slave quarters, speak with Susan. When she asks for help, choose the option "You're a slave. Deal with it.", then return to Grouse to collect your reward again. Grouse will think that you captured her again. You can repeat this for 250 caps and a slave collar each time.

Note: If you capture all four slaves and tell each one to "Deal with it" then talk to the man after the initial time, he will give you another 250 caps for each one. This makes it much easier to get high amounts of caps instead of running back and forth with just one.