In a new home purchase, once your loan is approved by your builder lender, or you submitted a approval to the builder via the sales office from a lender of your choice, the loan contigency is consider removed ( you don't need to sign anything like in a resale). Once your loan contigency is removed, unless you have other contingencies stated as part of the purchase agreement, legally the builder can keep your deposit if you want to back out. IN other words, you are NOT stuck with any loans (from the builder or your lender), it just means you are "stuck" with the home unless you are ready to walk away from the deposit. If the builder is nice, and you have special circumstance like divorce, lose of job etc, you might get a break. If you have job transfer, they won't give it back to you either in this market.
You don't have to make a decision on the loan until closer to closing. I still have a hard time to believe this entire 75K is tied to using their lender. You might want to call them or read the agreement to double check. Who is the builder? Usually incentives towards upgrades are completely seperate from the incentive towards using their lender. There is a seperate appendix you have to sign stating that you have no obligation to use their lender, however if you do use their lender, the incentive is x amount.
Here is my suggestion for you: first, do you homework, make sure you understand exactly how this incentive works. Secondly, call your builder broker and tell them their rates are too high, and you want them to broker the loan out. Usually, they can broker out the loan to most major banks (at least that is what I was told by my builder lender).