I put stuff on sale all the time and a lot of mine have best offer on them. It still works fine. Why would you take the best offer function off just because an item was on sale? I find most people just use the buy now option because it's already discounted, so there's no need to make an offer.

As catspjs said, if you make a change to a listing you can't put it on sale for a period of time, I'm pretty sure it's within 12 hours.