It sounds like you want to go under contract but don't have your loan arranged.
If I'm reading that right, the seller's request is more than reasonable. He wants to be sure that you can actually perform to the contract - and at this point, you cannot as you don't have the funds necessary to close.
That's because you went out of order. First, get your pre-approval - and not a garbage pre-approval from a Rocket Mortgage (or similar), where you fill in the blanks on an application and get an almost automatic pre-approval.
The reason I say those are garbage is that the lender hasn't reviewed any of the supporting docs, like bank statements, pay stubs, etc. Nor have they verified employment or even done a hard pull on your credit.
Once you have a proper pre-approval, then go shopping and put a property under contract.
The seller is smart. They are not going to let you tie their property up for 30 days without knowing that you can actually buy the property.
You have your marching orders. Now go get a proper pre-approval!