Here's my take:
You mention that you're ALMOST DONE with your homestudy. In our experiences, UNTIL that homestudy is complete and signed, few (if any) agencies/attorneys want to deal with you. You could understand that, simply because they have to have that paperwork should anything come up, KWIM?
Secondly, not ALL agencies/attorneys want BIG $$ up front; and, once your homestudy IS done, why can't you also submit to any other agency/attorney you want to? A lot of us deal with more than one agency/attorney in trying to find our children. It's not unusual at all. In fact, I'd be leary of any agency/attorney who told you that you couldn't deal with more than one in your search! (FWIW, I just had an email from one agency who listed in their paperwork that they would ONLY allow clients to work with them...and NO OTHERS. THAT was a HUGE red flag to me, and I promptly checked them off as a resource for us! )
While it's good, I think, that you're being cautious, this attorney's attitude may be this way for the reason stated above. Also, that 'cautiousness' is a good thing when checking out any agency/attorney, IMO.
Good luck; and I hope your homestudy is completed soon!
Sincerely,
Linny