Sounds like you need to stop working overtime.
At one of my previous jobs, I was paid hourly. It didn't take them long to make me salaried when they realized that my job required me to work 60 hour weeks and that they were losing around fifteen or so thousand dollars a year in overtime. I took the pay cut there in exchange for me no longer having to keep track of my hours, not needing to take breaks or lunches anymore (which are legally required for hourly employees where I work, but I prefer to just work the whole day through and was constantly getting written up for it), etc. But, at this place, I still have to track my time in detail since we bill clients. So I get no benefit like that here; at least I don't have to take breaks or lunch. But I don't even get dental.
When it comes time to do this particular work, make absolutely sure it's late, and in the middle of something else that's higher priority; let the sales rep squirm. Then make sure that most of the work does not fall in the commission-generating category.
That will slow down this sort of idiocy.
Yeah; an issue here is that once a client gives us their money, they immediately become priority #2 to clients we are trying to sell stuff to - it's an unspoken company policy. A $50K potential deal is a higher priority than the $100K deal we've already been paid for. And when we strongly believe a project will be a failure, we just ask for, say, an inflated 25% up-front and expect to never be paid the rest and for the project to be cancelled. Further, I have in my job description to make sure X% of my time is spent on billable hours each week. So there is a floor there I can't go below - since product support makes up a huge part of my job, and support is not billable (excepting in some rare circumstances), that really limits my options.
Likewise, clients who complain the loudest get more attention from sales and management than the really good clients who are great to work with. Clients who refuse to pay their bills get quick and excellent service (or quick and shitty, depending on how much they curse at my boss), while the ones who always pay on time and without dispute get put on the back burner. I even have some guy whose issue I've been putting off for over three months now just because he is so nice and I need to deal with the clients who are asking for refunds first, even though I'd much prefer to help this guy out. I don't get commissions for when I do a good job programming something, but you can bet that I'll be penalized at review time if customers start returning products (even those I didn't program).
I can really understand why they say executives tend to be sociopaths. They are incentivized in every way to either warp or abandon their ethics, or encourage people who have none. And, honestly, after you deal with your 5,000th client who calls you incompetent, stupid, claims you don't know what you're doing, makes insane demands, berates you at every chance, ignores your advice, blames you for all their problems, and so on, it's kind of hard to not start actively hating clients before you even talk to them and treating them as de facto enemies whose lives you want to make miserable. I never used to understand some people's attitude or behavior like this until I started doing customer service. I also used to find Dilbert funny. Now it makes me want to cry.