GM dealers arrange floor plan financing primarily with one of the large Canadian banks. This is in the form of a multi million dollar line and charged below bank prime. Each car is chattled and paid for. GM may assist in carrying costs by offering an interest free period to entice dealers to stock more. An amount equal to a certain number of days arrives to the dealer when the car is shipped. The manufacturer does not do this when there is a severe shortage of product.
The question of what a dealer sells a car for is up to them. It is the same for discounting. MSRP is a manufactures SUGGESTED retail price. A dealer can imply the manufacturer is supporting selling over list and the customer is aware they are paying a premium. In essence it is market price. GM does not receive any money back.
I respect the dealers that stick to list and even apologize to loyal long term customers they cant afford to discount because of the waiting lists. In most cases those dealers will prioritize customers they already have relationships with to avoid unfamiliar customers 'flipping' the car for 30 k more than they paid. It's found money the dealer did not take advantage of. If the car is flipped into the US, the customer will have an extremely hard time being able to order another one anywhere in Canada.
Lastly, when someone ships or sells to an exporter we all loose. GM Canada fights for allotment from the US when we are not a huge consumer. At best 10% of mainstream and far less in the sports car market. Once they receive that allotment it is distributed to the dealer network. Every time a new (less than 6 month old, under 12k) Corvette goes back to the US, GM concludes its one less we needed in the first place and won't increase our allotment, even reducing it.