The rezoning cannot close with you; it has to close with the buyer, which means your contract needs to survive long enough for him to apply as the owner or at least as the party in interest. One path is to convert the assignment into a double close where the buyer takes title at your A-to-B leg and then prosecutes the rezone himself, accepting that he owns raw ag land at full purchase price while he waits 90 to 120 days for a decision that may not go his way.
Take a parcel bought at $31k with a $15k assignment fee baked in, so the buyer is in at $46k before carrying costs. At 120 days, even a modest land loan at 10 percent annualized adds roughly $1,400 in interest, plus any application, environmental, or traffic study fees the county requires, which in California can run $5k to $15k before a public hearing is even scheduled. He needs to price that total exposure against his post-rezone upside before he commits, and you need to know whether his $46k offer was priced assuming the rezone was already done or still pending.
The seller extension is the lever you actually control right now. A $2k to $3k non-refundable extension payment for 60 additional days is a real number you can put in front of an antsy seller, because it compensates him for the wait without making him feel like he is just holding your option for free. Whether the buyer funds that extension payment is where you find out how serious he is about the rezone path.
Does the buyer have a California entitlement attorney already, or is he expecting the rezone to be a simple administrative approval?