I actually had an issue with the minimum allowed amount to finance with when I was looking for my Focus. One dealership where I had found a red '09 SE coupe said I could only finance a minimum of $5,000 through my credit union. I went to another dealership where I ultimately found my blue '09 SES coupe and they were able to finance me for only $2,500 through my credit union.
I assume that's what you're referring to and hope my experience helps you decide. Both instances were with my '12 Mustang as a trade-in as well (I still owed about $10k on that).
I am currently going through this right now. the minimum allowed amount you can finance for the purchase of a car is, I believe dependent on your credit institutions rules. My Credit union allows auto loans on used cars for as low as $5,000 for 36 months. But the other Credit union I went through said their minimum was $7,500. (this is not dependent on credit, but the least amount they will loan on a car)