I purchased a Hampton Bay ceiling fan which indicated it would work on a sloped ceiling. As the final step of installation the installer found that due to the angle of the ceiling, the collar that slides to the top of the extension pole and covers the wires in the box will not accommodate the angle. So now the collar is sitting on top of the fan and there is a big hole in the ceiling full of wires. Is there a solution?

Maybe a vaulted ceiling kit, or a carpenter to give a flat area.

You can look for "collars" that have a longer slot or larger hole in them to accommodate the angle. Big box stores usually have a selection to choose from. You can also look into enlarging the slot/hole in your existing cover. But before you do this, you need to ensure your mount will allow that much of an angle. You sure don't want that thing coming down on you.

Do you know the angle of your ceiling?

Steeper ceilings may require a kit.

Frequently asked ceiling fan questions - Ceiling Fans N More (http://www.ceiling-fans-n-more.com/ceiling-fan-faq.php)

A vaulted ceiling Kit.
http://www.littmanbros.com/store/item.aspx?DepartmentId=414&ItemId=36448

That one will work, there are plenty of others.

The mentioned kits may work but it isn't too hard to build a triangular box that attaches to the ceiling and matches the roof slop so you have a flat bottom. Just picture a speed square held up to the ceiling, the box would be shaped like the speed square.