[DoubleR]

And don't forget the BVD test also. Super important not to spread diseases.
Be sure your cattle are up to date on all vaccinations prior to introducing the new bull. You might want to check and see what vaccination and worming the bull has had recently as well. He can easily bring in parasites that can wreak havoc on your herd.

 

[DoubleR]

Don't word of mouth and hand shake this.
Write up a contract and have both parties sign it. These days are different than the olden days when a hand shake was all you needed.
If you can't afford to replace the market value of the bull don't borrow him. As said above if he gets hurt or dies while with you that's on you. It's unfortunate we live in a day and age like this but it is what it is.
And never assume "He's a good guy. If something happens we'd work it out."