I think anyone would be salty if they sold the rights for a $10K flat fee and had missed out on $16M in royalties (by Sapkowski's estimate). He does seem to mostly blame himself though:

“I was stupid enough to sell them rights [to all of my novels],” he told Eurogamer. “They offered me a percentage of their profits. I said, ‘No, there will be no profit at all — give me all my money right now! The whole amount.’ It was stupid. I was stupid enough to leave everything in their hands because I didn’t believe in their success. But who could foresee their success? I couldn’t.”

I think he's de-salted though. They settled and renegotiated a licensing agreement a few years ago. His relationship with CDPR seems fine now, even if he remains fully committed to grumpiness:

"The contracts between me and the game people are excellent right now. Let's hope it stays that way."

Although CDPR's Adam Badowski says he's secretly a lovely dude:

"He's a very intelligent guy," said Badowski, "and as a writer, he created kind of a persona. In person, he's a great guy. Very knowledgeable." In fact, Badowski says he hangs out with Sapkowski regularly: "We are friends. I visit Andrzej regularly. We go for dinner, have great conversations."