In Pokemon Red/Blue, can you buy the bike before you get the voucher?

It is impossible, the bike costs 1,000,000, but you can only hold 999,999.

The Bike Shop has a store in Cerulean City, which sells Bicycles produced by the company. Designs range from a plain city bike to fast mountain bike; however the player only ever gets one: the Bicycle. It can be bought for 1,000,000, or exchanged for a Bike Voucher. However, as only 999,999 can be obtained at any time, there is no way to buy a Bicycle without a Bike Voucher. The Pokémon Fan Club Chairman in Vermilion City will give out his Bike Voucher, making this the only viable way to get the item.

Source: Cerulean City