You should call your attorney again and again and agin if necessary and get an answer to whom this fee is going to, how much and why if you have a mortgage contigency clause in your contract.
I would ask . . . . (1) were you pre-approved or pre-qualifed before signing a contract for sale? (2) were you represented by an agent before signing a contract and did that agent review/solidify the preapp/prequal and advise you of the pro and cons of entering into such a contract based upon the approval (devoid of the non-existent co-signer?
I'd have to agree with the previous posters that at this point, it is a matter better addressed by an attorney. If your attorney is non-responsive, then send him/her a letter stating such and exercise your rights to seek alternative counsel without obligation and your need to receive an immediate response. If he/she continues to be non-responsive, then with this documented correspondence/coupled with his/her lack of response should warrant the need to seek alternative counsel devoid of financial obligation to your current counsel.
I apologize for any inconvenince this experience may have caused you.
Love and Peace,
Francesca, ePro, SRES