In the abstract, you can elect to choose a fair market value or the lower of cost or market method for valuing inventory.
Where you have a lawyer who's incurred expenses, costs in order to earn their revenue, the lawyer can measure that file under work in progress, in respect to that file, at the lower of its cost and fair market value. I believe in your example it was $10,000 of work in progress at the end of the year. Let's say they have $10,000 of work in progress at the end of the year, and they have incurred $6,000 of costs, for example, to pay an associate on the file or whatever is properly allocable to it. They would value, for tax purposes, at the lower of their costs and fair market value, so the costs, which would be generally deductible, would offset the income from the work in progress. But that's just in an abstract case.
I recognize you'll have more than one file in a practice, which brings us to the second point, which is that it essentially represents a deferral. Think of it in the simplest case, where you perform services in one year and then bill in the next. Let's say, to make the example simple, you have just that one item worth $10,000. Under the billed-basis system, where you get it included in your inventory when you send out the bill, you could add the costs in the first year but then you'd have an income inclusion in the second year, where the first year's income was deferred to. You will pick that up in the second year.
In that year, if you take on another file with a contingent amount to be paid after the end of the year, so you bill at the end of the year. Let's say that's of comparable size of $10,000, then you wouldn't bill for that in the current year, your second year, you would bill it in the third year. If you look at the tax results in the second year, you have $10,000 that has been deferred from the previous year, and then $10,000 deferred into the next year. You can see that you end up in roughly the same place as if you had, at least in respect of that year, been taxed on an accrual basis.
If you're deferring income from year to year every year, and certainly taken across the industry, it's primarily a deferral benefit where one inclusion from a previous year would ordinarily tend to offset, to varying degrees, income deferred to a following year. You have that averaging out as well that mitigates the impact of the change.
To summarize and answer your question, first, if you have a real contingency, perhaps it's not included under the case law and general rules. Second, if it is going to be subject on an accrual basis, there's the option to use the lower of cost or market method. Third, if you have a number of files, where some are billed and some aren't, over time you would expect the other deferrals to even out.