I'd get your EM money back & then kill the first contract. Hopefully - your initial contract wasn't so screwed up
that TB had a right to keep the EM even if their in-house lender denied funding - for whatever reason.
Then - make an offer for 900k (as I assume you meant 1.5 million list before) :-)
The issue I had in mind relates to C-to-P loans when it's time to convert to permanent - secondary market typically requires a warrant form the lender stating that the value hasn't dropped since originating the loan. Many folks have been stranded by that little issue since the market tanked, and the lenders don't divulge the problem; instead they try to beat a huge cash infusion out of the buyer to make up the difference.
Unfortunately, outside the lending realm I'm not qualified to offer suggestions. It sounds like much will depend on your contracts with Toll Bros and if your attorney is able to link the purchase contract with the mortgage commitment. Partial performance and consideration might be the keys to conditioning one promise upon the other.
Knowing builders, I'll bet your attorney finds a way. Buildersof all sizes seem to be sloppy in keeping a clear separation of relationship between the sales and mortgage divisions.
Good luck to you. And keep us posted.