So if the tax is less than $7,500, nothing has to be paid back, by either the dealer or buyer.  But if the buyer's income exceeds the AGI limit, they take a hit. They must provide the dealer with,

"An attestation that in the event you exceed the applicable modified AGI limitations, you will repay the amount received as an addition to tax for the tax year the vehicle was placed in service."

https://www.irs.gov/newsroom/topic-h-transfer-of-new-clean-vehicle-credit-and-previously-owned-clean...