Your $7,500 tax credit is supposed to help pay for the interest. 5% on $40,000 (for easy math) for 36 months is roughly $3,150 for BMW Financial. If the dealer decided to do $43,150 (above MSRP) then your 2.99% financing would be about $2,000 in interest over 36 months for a total of $5,150 (above MSRP + interest).
I'd much rather see a higher interest rate than finance/purchase above MSRP, but it also means the buyer needs to have the cash available. Financing a Signature 2.0 at $34,225 for 36 months at 5% would be a total cost of $36,927 which already exceeds the Iconic 2.0 $36,700 MSRP outright cash purchase.