I think this maybe a duplicate ?
It sounds like you may have rec'd a short sale approval letter that states that. If so, good it got approved.
This is typical language lenders put in the short sale approval letter. Yes, they can do it if you are agreeing to proceed with the sale, (I'm basing this thinking you have an approval letter).
But, typically they don't, not lately anyway in my closings of short sales. If an extension is needed the listing agent requests it and hopefully asks for no extra fees.
But, In the past over 2 years ago, I did have a lender that granted the extension, but did charge about $200 extra.
If Your lender is the reason it takes to long to close, make Your lender pay the fee if any.....just a suggestion