offer it for $25k and wait them out. domainers always say "We've turned down offers of 10x that" whether you offer them $25, $2,500 or $250,000. It's like domainer standard practice or something. Domain buyers know that so the first offer is often their idea of a lowball.
seriously, what kind of bum offer is that. $250 up front. This guy is a lawyer. Lawyers don't work for less than $250/hour. $250 doesn't buy you doggiepooperscooperstoreinidaho.com for goodness sake. Tell this clown to come back with a real offer unless the domain is a complete POS
Is it legal to sign a contract and lease/rent the domain for a monthly fee? If the app takes off, you renegotiate for far more. If not, then whatever...?