Not sure you can under your agreement.
Most lawyers will give you a free consultation if you ask for one. We do this all the time, but remember that our time is valuable and you should stay with the people who help you the most.
It would take a review of the contract to give you an answer. If there is nothing in it which compels him to sell the products or makes an alternative payment arrangement, it may be a problem for you.
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Have an attorney review the contract for you. Hopefully, there are provisions which explicitly, or at least implicitly, put a duty on the buyer to operate the business in good faith, continue to promote your products for sale in the relevant marketplace, not act to impair your expectancy under the contract, or something similar. Provisions like that will give you the ability to persuade Jerry to keep making reasonable, good-faith efforts to sell your products.
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I agree with the other attorneys - you'll want to have an attorney review the contract for you. One may do it for free as part of a free consultation, or may charge their normal hourly rate. Either way - it's worth it. I'm hoping there's something in the contract that deals with the possibility that Jerry may not sell anymore products. If the contract is silent on the issue, you may still be able to recover if the spirit of the contract was to make sure you got your $20k. Best of luck!