Not really, your best bet might be to include a cryptographic signature of the document with the signature. This would be the only way to effectively prove exactly what document you approved.

On Excel 2010, the review tab has two major options under "Changes" labeled "Protect Sheet" and "Protect Workbook". You'll have options for allowed actions under "Protect Sheet" and workbook layouts under "Protect Workbook".

Keep in mind that this is not the hardest thing to crack. You can easily crack such a password in no time from a google search.

You'll still have to format the cells and select "Locked" and/or "Hidden" under the Format>Protection tab for each cell. This property is slightly buggy, as copying cells in a locked worksheet/workbook also transfers the value of this property.

If your boss is that predictable, why don't you add some extra cosmetics for protection, such as visual basic code that doesn't do anything through the developer tab, data in cells that are way out of the bounds of where he'd normally look (column/row 100,000), or a hidden worksheet -- or all three .. and even more. People who claim others' work as their own are rarely ever capable or intelligent enough to double check for these things.

EDIT:

An md5 hash would also get you a long ways towards verifying the state of the file. It's nearly impossible to have an md5 collision on accident.

MOAR EDIT:

Here's a site that will supposedly generate md5 checksums locally without uploading.

http://onlinemd5.com/

EVEN MAOR EDIT:

Apparently SHA3 became an official US standard last year, so you should use a SHA3 hash if you decide on such a thing.