Contract mistake on FHA assumable loan

I'm in binding on the purchase of a home, with an addendum that includes the interest rate and payoff amount of the seller's existing loan. This information was also represented in the listing and verbally, and obviously was a huge motivator in going with this versus other options.
However, the company I'm working with on assuming the loan has confirmed a higher interest rate by 1% and a higher payoff amount by a several thousand. Both of these make the deal worse to the tune of $150/mo more and $40k of additional interest owed over the life of the loan.
What recourse do I have as a buyer in this situation? I assume I can recover my earnest money due to the seller refusing / being unable to honor the terms of the contract.
I'm also trying to determine a fair calculation of a purchase price reduction to compensate. Is there anything else I should consider? Any other leverage I may have, should the seller choose a position of "take it or leave it"?
Thanks in advance for any advice or help here, this seems to be quite a strange contract edge case given how rare assumables are.