Well, it sounds good for you if you are not going to raise any extras for him just give him your 'naturally-occurring' leftovers...
...BUT, it is not clear to me that's going to be what this guy is looking for. It sounds to me like he may be trying to sell ready-to-lay pullets, which generally involves having contracted with someone else to buy a certain number of a certain breed.
The only other thing I can think of would be for 'ethnic' food buyers who prefer non-CornishX for their meat birds, in which case your leftovers might be ok for him.
Of course I can think of a whole bunch of less rosy possibilities, e.g. training fighting animals, but I won't go there.
(BTW, I'm confused now -- $200 per month, or $200 per four-month cohort of birds?)
I guess basically it comes down to finding out whether there is something mutually agreeable that you two can settle on. I'd still suggest a contract if it involves you raising any more birds than you would normally do.
GOod luck,
Pat