You say "Lawyer stated we could not keep any of the downpayment." Was that your lawyer or the would-be buyers'? If it's the buyers' lawyer, what does yours say? (Don't trust a lawyer representing the other side.) If it was your lawyer, then I defer to his/her judgment. Still . . .
Others will point out that your agent should have made sure at some point in the process (the sooner, the better) that the buyers were pre-approved or at least pre-qualified.
Unfortunately, it's virtually impossible to force someone to buy your home. And though it's not a pleasant thought, your best prospect is still the #$%$# buyers.
However, you also might want to re-examine your relationship with your Realtor. I obviously don't know the details, so I can't make any suggestions. (And even if I did know the details, the Code of Ethics would severely limit what I could say.) But perhaps there is reason to believe that another agent might have handled the process differently. Just something you may want to think about.