Does a buyer rebate affect the appraisal?
No. The appraisal is about whether the home is worth the contract price, and a rebate does not change the contract price.
What the lender does look at
US lenders review all credits from interested parties, including agents, to make sure the real price is what it seems. That is about the credit, not the appraisal.
When a rebate is converted to a price cut
If your rebate is larger than your lender allows as a credit, your agent may ask the seller to reduce the price instead. Then the appraisal is against the lower price, which can only help.
In Canada
Because the rebate is paid after completion, Canadian lenders and appraisers are rarely involved at all.