The Seller’s Lender will generate a short sale approval letter with the terms of the sale.   That letter will contain a paragraph stating that they want receipt of proceeds (funds) by a certain date.    Some Lenders state that “if there is a delay in the closing and/or disbursement of the proceeds, the offer may become null and void”.

If you can not meet the deadline to close, you can ask for an extension.   However, there is no guarantee that the Lender will honor the extension.