If you buy the car for cash, that transaction is complete and won’t be reopened by any car dealer.
To ‘finance’ a car you paid cash for, you can get a loan or line of credit from the bank. It won’t be favourable though. The best interest rates and incentives are always through the dealers themselves.
Keep in mind that incentives from car dealers are usually like an a la carte menu. You can choose a special cash incentive, a special finance rate, or a special lease rate.
I assume you are trying to get the cash bonuses (which are indeed usually the best) and then also get the special finance offer. You’ll have to choose.
Try your bank or CU. Let them know ahead of time, as they may be flexible in offering you the new car rate financing as opposed to a used car rate refinancing. Could be a slightly better rate. Exactly a year ago, Tesla sent out those end-of-quarter text messages about having cars matching orders. Of course, it was just a fishing expedition to find out who could take a car on short notice. I immediately went to my CU to start the financing process, but they said they could finance me a week later, when I got the car, with VIN, etc., and I could still have the prior month's new car finance rate, etc.
Not sure if it will work with Tesla, but I remember years ago my first car was bought on a weekend too. I wrote the dealer a check for the full amount but asked them to hold it (don't cash it immediately), then talked to my credit union on Monday who promptly gave me a check to give it to the dealer. Then the original check is returned to me.