What is his total price? Do you have most of it, but are 600 short? Maybe you could pay down on him what you have in cash and arrange for a payment plan for the missing 600.
One thing to keep in mind. Buying the horse is not the last time you spend large amounts of money on him. If he has to go to the vet, etc. you need to be able to come up with the money.
If you are already struggling to make it now, you might consider postponing horse ownership until you can ensure that you have the funds for emergency medical care just in case...