I once did this and expensed the miles at the documented price it (would have) cost to replace the miles by buying them from Delta, under the assumption that I'd use my miles to save the company money and the company would buy the miles back for me. However, I proposed this arrangement well in advance to the person who would approve the charge. I'd never try something like this without getting approval in advance.

In the end I didn't use the reimbursement cash to replace the miles, but I think the "replacement value" is a fair and most importantly, documentable and defensible means of valuing the miles, so long as it saves the company money at that valuation.

More recently I've been using a lot of discount business fares, and it's usually impossible to save a lot (or even any) money over that option by using an upgradable Y fare plus FF miles at the "buyback" value.