The assignee backed out of the deal but won't sign paperwork

[Mahmoud Y. Elhalawany]

We assigned a contract to a buyer who backed out of a deal and now we have a new buyer lined up. Unfortunately the old assignee is being difficult, our attorney can't give us legal advice because technically now she is his client as well. I need this woman to send in an informal letter stating that she is in fact terminating the contract, but as I said earlier she is being difficult. If this deal falls through as a result I am going to seek legal action against her. What should I do?