Why would a lender NOT approve this?

[Tom Gimer]

Short-sale approval obtained a while back... it's Friday, closing is Monday.

Purchaser had something come up and now wants to buy a different property and have his "friend" slip in and close on the deal. 

Assuming the right bank contact can be reached (and proper assignment, etc. executed), why would the shorting lender NOT approve this? Perhaps I'm not thinking of something.

Thanks in advance for any quick feedback.