I know me and my husband discussed this serveal times while watching movie,so come to conclusion that Dr Faclier was behind it. At first I was uncertain it was Dr Faclier but after thinking about it for while,it begain to make lebih sinse.
We know Dr Faclier ( Shadowman) had been spying on both Tiana and Naveen from the begaining of movie. It did had every intuitions of trapping both of them to get his debt payed off. Naveen was a prince,Tiana was trying to get restrunt and Charoltte (Tiana's best friend) was rich. And here anda go there your answer to your question! So all these piceses fall into place and it all leads to the shadowman!
I thought maybe nobody had outbade her -- the Fenner brothers really didn't want to sell that building to "a little woman of [her] background." They probably went along with it because they didn't expect her to ever come up with the money to actually buy it.