Modern perceptions of "slaves" are very different to old testament reality. People hear the word "slave" and picture whips, chains and cotton-picking negros. Forget all that.
In the early stages of the Old Testament, MOST people appear to be slaves! Think about it. Abraham had 318 slaves (usually translated servants but it's the same word) who rescued Lot with him, and this was all the men of his house. Why were they slaves? Because people simply didn't use money much. You wouldn't get paid a daily wage, instead you'd be attached to an employer (tribal chief) and work for the tribe in return for your food and everything else you needed. And you would be committed to the tribe for an extended period of time. Technically, the arrangement a slave has. But it didn't mean they had no say in anything in their lives.
A great illustration of the actual relationship between a master and slave is shown when Saul and his slave/servant went hunting for his lost donkeys. They wanted to give a gift to Samuel, and discussed together what to do. Saul had no money on him, yet the slave did have money, so they used that. How did the slave have money, and the master had none? Why do they seem to be discussing what to do as near-equals? Because things really weren't how you imagine. The slave was a member of the household, a trusted part of the family.
Don't assume that just because someone is in an employment relationship where they are paid in food and clothing instead of cash, and have a long-term or permanent contract, that they are completely oppressed and have no say in anything. That is really jumping to conclusions.