Personal assets used for one-time business expense

1. Expense a personal asset-

• you can depreciate an asset used in the business that was yours and mark that it was converted to business use.
• If you bought it new in 2020, using it for the business, you are able to expense it.

2.Yes, you can bill a customer anything you want for whatever price they are willing to pay. 

3. Backup for tax purposes, would be the purchase receipt if new. If interacting with your client, refer to your agreement.

I understand that I can depreciate an asset converted to business use.  This item is a $600 item that was used only once.  It doesn't seem reasonable that I would convert the entire cost over to an asset for a one-time use. 

But maybe the usage doesn't matter and it should be considered a business asset now?  I thought it would make more sense to "rent" it to my business.  The going rate for renting one of these from the local rental is $50 per hour.  Is it ok to rent something to your own business?

Just bill your customer for the use of the tool.

You cannot rent it to yourself as it would be tax neutral. The rental cost to your business will be rental income for you.