Normally when a bank asks for a Promissory note it's an interest free note payable over 10yrs., at $100 / mo for 10yrs, would only cover $12,000.
The listing agent needs to fight back to the bank, resend in the state of your sisters' financial information. Prove she doesn't have the money to pay this amount back. OR talk to the buyer & have them offer a CASH contribution now of 10% of the $19K to see if they'll settle & release now.
Stand firm that promissory note is out of the question & try to see what their number is to settle now in cash, which will have to be paid for by the buyer, if this is a 2nd lien, that cash contribution will have to be known to the 1st lien holder as well. The listing agent must show the 1st lien holder that it is still in their best interests to allow this and the NET to them is still better closing through the short sale than them taking the property to auction or it reverting back to them for future REO asset.