It is important to note the Rabbi is not a party to the contract which is between you the buyer of the chametz. The Rabbi is only the messenger (shaliach) who acts on your behalf for this contract. As such, should the Rabbi become incapacitated the contract remains valid since it is between you and the buyer.
If the buyer becomes incapacitated, then he will not pay and the chametz will return to you automatically.
See here for a sample form empowering the Rabbi to sell on your behalf.
The OU also mentions here that
Contrary to a widespread misconception, Rabbis do not buy the chametz.
The individual participating in the sale appoints his Rabbi as an
agent to sell his chametz to a non-Jew on Erev Pesach.
Second, at least in Israel, contracts are normally written the other way around, i.e., "if you don't pay a million shekels (actually billions or more in the case of the public chametz sold by the Chief Rabbinate) by that date [right after Pesach]) then the chametz that we sold you returns to its original owner".
See for instance this model contract that states explicitly (bottom of first page) that
The Buyer must pay the Subsequent Rent and Subsequent Payment
(together, $100,000) on or before the Ending Time. If the Buyer
fails to do so, then, at one minute after the Ending Time: (a) the
Lease terminates, (b) title to the Transferred Chametz is
automatically transferred to the Seller and (c) the Buyer is
relieved of the obligation to pay the $100,000.
See a similar, funny, contract here