You did not ask the most important question (yet, you should now), which most of the answers just assume one way or the other:
"Is the company unable or unwilling to pay your full salary?"
If they are unwilling, you can begin a discussion about your performance, or the performance of the product. The former is a serious discussion; the latter is not, because unless you are the responsible product manager with control over the marketing and R&D budget, the performance of the product is not your responsibility.
If the company is unhappy with your performance and believes that you are not worth the salary they negotiated, you can accept or push back. Pushing back is very simple: You refuse to have your salary cut. You remind them that negotiations are part of business and if they think they negotiated badly, they are free to cancel the deal entirely (fire you) or offer to change the deal (cut your salary), but only one of these options does not require your agreement. And why, exactly, would you negotiate well and then undo it all?
If the company is unable to pay, it is bankrupt. Not in the future, not potentially, not maybe. The definition of bankruptcy is the inability to pay your due bills.
In that case, I can only support every other answer that recommended to leave. Collect whatever they might still owe you (travel expenses or such things) immediately and be out the door before the whole thing collapses.
There is also an important personal decision you have to make: Will you ever accept a cut in your salary? It helps to have principles. One of mine is that I will never do the same work for less money, and the offer itself is enough for me to quit. I was in that situation and I could almost certainly have successfully fought the attempt (had connections). I didn't. I asked for time to think, then never mentioned it again, found a new job and made sure to inform the top boss (who was very sorry to lose me) about the exact reason I left and the person responsible for it. Fun fact: That person doesn't work there anymore. :-)
(Footnote: For an actual pay cut, the answers saying that you could ask for fewer hours would be on point, but by calling it "retaining", they've cut off that option for you, possibly intentionally.)