If there is a signed and *enforceable* contract, they cannot reduce their agreed-upon assistance, even if it was overly generous under the circumstances and reduces their profit or costs them money.But if the lower appraisal causes the home's price to have to be reduced (e.g. to qualify for financing/mortgage), then you, as the buyer, have a choice: either find a way to purchase the home at the originally agreed-upon price, despite the financing difficulty caused by the lower appraisal (which may mean coming up with more money out of pocket), in which case the contract (including the amount of assistance) remains in effect; or agree to renegotiate the contrat for a lower purchase price, so you can get the financing you need, in which case, as a condition of the renogotiation, the seller can insist on less assistance. Basically, if you don't need anything from the seller (e.g. a price reduction), you can hold to the contract as is; if you do need something, they can put conditions on granting it, like giving you less back in assistance.