Sometimes clients contact you to cancel your work before it's finished.
In some cases they don't want to pay anything for the work you've already done, because they no longer need it, and they don't think they should have to pay for something they don't need.
Well, that's their perspective, which is only one side of the story. Your perspective, which is the other side of the story, is that you've given them your time and skill, and you have a right to be compensated for it. My opinion is that you'd be in your right to charge them for that work (minus polishing if it goes straight to the bin and won't be used) unless you can easily sell the same translation to someone else.
But what I really wanted to post about is when you consider charging them for the 100% anyway.
I believe it would be unethical to not do the remaining part of the work but still charge them for it — unless with their consent.
Simply put they have paid for our time. We have sold it to them. It now belongs to them. We can't keep it to ourselves or sell it to someone else any more. One doesn't own what one has already sold. One can't sell more than one actually has. One can't sell the same thing twice — for example we can't just charge that one client for a full day's worth of work the client will not actually receive and then spend that day working for a different client to get paid for it and profit from the cancellation by doubling our earnings for that day.
We could offer to translate something else for the same client within the limits of volume or time already paid for by that client. Or we could credit the time or volume against future projects.
But not charge two different clients for the same time.
Example: Suppose you're paid on a per-day basis for 10 days. The client cancels on the 8th day. It's easy to see that charging the client for all 10 days but spending the last 2 days working for a different client and getting paid for it means getting paid for 12 days after working only 10. This would be ethically sound with the first client's freely given consent, but not as the translator's dictate. Without the client's consent it would be like having one's car stolen and then returned but still claiming compensation from the insurer for the full value of the car (rather than lost enjoyment for some time, cost of checkup, repairs, cleaning etc.).
It isn't really different when the billable unit is word or page — only harder to visualize.
However, even if we simply take an improvised holiday, do completely nothing but just rest and regenerate our strength, then we're still keeping that time to ourselves. It isn't really fair to still charge the client for it and make the unexpected holiday our gain. That would be an unduly translator-centric perspective — just like it was unduly client-centric perspective to pay 0% after cancelling the work midway through as no longer needed.
We could also spend such time doing all sorts of things that need to be done anyway. Invoicing, taxes, administration, marketing, CPD, chores, even house chores — as we need to do those some time or other anyway. In fact, we'll probably end up doing something like this.
Notably, by shuffling our schedule around a bit — moving things to different days or hours — we're making ourselves more available for any future assignments, for which we'll be paid. This means no actual loss is suffered, only the inconvenience of having to adjust the schedule.
Perhaps the inconvenience of having to change our plans should be compensable, but it simply is not the same as actually losing the time we were expecting to sell. This isn't fresh meat or fish rotting because the buyer balked.
Thus, if we also forced the client to pay for our time, we'd effectively be profiting from the same time twice, which is little different from selling the same time to two different buyers.
Back to the insurance example: If we had insurance against cancelled contracts or other loss of work, the insurer would expect us to mitigate the damage by at least looking for different work, or, as an absolute minimum, not turning down viable offers. The insurer would only make up the difference.
The core principle of compensation is that one isn't supposed to profit from accidents. It isn't free lunch for the victim.
Bottom line: Having a PO or a signed contract is not a licence to be unproductive, or to sell the same time to two different clients.
However, I would also add to this that it's not okay to just go ahead and finish the work and force the client to pay for it, either, for very similar reasons.
We can find different work. We can rearrange our schedules. We can do some CPD, marketing or admin, or chores, or take an evening off today rather than tomorrow. This doesn't inconvenience us in any serious way.
Hence, let's not use POs and contracts as weapons of extortion.
Exception: The situation is different where at the same time:
- we had to forego alternative work in order to accommodate that client's work; and
- we needed work to pay the bills, can't find other work on short notice and can't really accommodate a holiday etc.
— in which case we shouldn't have the choice made for us by a random circumstance to have a holiday instead of working (for example). That would make me see it in a different light, ethically.
But normally it would be better to charge only moderate cancellation fees beyond payment for work actually done.
One also needs to remember that ruthlessly profiting at the client's expense is not compatible with the ethics of learned professions such as doctors, lawyers and other advisors.
Would you be happy if your lawyer or doctor charged you for previously agreed work you no longer needed, if he could easily take on another client or patient or spend the time managing his own affairs to free up more time for paid work tomorrow or later this week?
 After finishing the translation in the normal course you would proofread and revise it on your own, then apply some edits etc., which is a time-consuming process. Hence, 50% of translation alone is not 50% of the whole job when you apply 0% of the final polish that was normally expected. In other words, the percentage you are at in translation alone is not the percentage you are at in your whole work.
 In some cases, which will typically be translations of published works, you may be in a position to finish the same work for a different client, for example a different publishing house. An argument could be made that you should give it a try before charging a cancelling client.