Agency: "Why don't you do more free work for us? When you do that we can give you better jobs for money?"

Me: "so if I do free work for you, I get to do paid work as a reward? ..you know that I am a freelancer, not your employee, don't you?"

*agencysulking*

Hello everyone :)

I left a job a while ago (8 month maybe a little more), before me alot of the team left, and the lasts ones left after me.
They hired back an ex teammate from years ago (he actually started the POC), but he doesn't do php so much and don't know symfony and he's alone. I'm not either (i don't like php), i was doing python and admin sys for them, but i saw the project going/evolving for two years, so i can help them.
They contacted me a week ago, asking me some help. I said yes, (because i believed in the company and i'm too nice i guess), so i spend two days making a new script to setup the environment and serveur and also had to do some package update on the project (late shit with pear php apparently).
I don't have any way to make a bill, i don't own any company. So I'm not sure what i should ask for money, and if i should keep helping them.
(it has been my first serious real job, and i put some money in the company that i took back).
Should i keep helping for nothing even if it's only few hours the month or should i change this situation fast?! (already worked 20h for them, and the boss a nice guy)

Thanks devRant