On the one hand, you could structure a deal on the basic theme of bringing the other agent on board to your S Corp as a senior-level employee, while you retain ownership of the shares.
Set him up with a base salary plus a bonus plan, structured along these lines: The net profit after expenses is allocated in a tiered fashion...
First, he gets some base salary.
Second, you receive some agreed-upon amount, for some agreed-upon length of time, where this payout to you calculated to mimic the amount and duration of the installment payments you'd have received had you actually sold the biz to him.
Finally, he receives any residual net profit.
Further, let him make the decisions and run the show (subject, of course, to your final approval---this would be a protective mechanism which keeps him from ruining your book and then walking away, leaving you with no business base to start over from).
So in a purely technical sense, you haven't sold the S Corp. But as a practical matter, this other agent is running the biz and receiving all of the net profits (less the installment payments he'd have been making if he had bought the company), just as if he owned it.
But on the other hand (and there's always another hand), there are two things you'd have to be very careful of. For one, check the contracts / agreements you have with the insurance company extremely carefully. Look for any language in there by which they might have the ability to declare that your arrangement constitutes a de facto sale of the S Corp, and hence a violation of the agreement. An attorney's advice on this matter might be coin well spent. (Also note that this arrangement does NOT offload any ownership liability burdens and responsibilities from you to him, the way an actual sale would, so take that into account.)
Second, research the tax law which applies in your jurisdiction to see if the taxing authorities might similarly have grounds for declaring a de facto sale of your corporation, based on the underlying economic substance of this arrangement. Advice from a tax expert in your area could be valuable, as a de facto sale determination would likely hit you with some ugly tax consequences you hadn't counted on.
But if these red flags are carefully vetted and deemed acceptable, maybe an arrangement similar to what's described above would accomplish your objectives and his, until such time as the insurance company would greenlight him for a bona fide purchase of your company.
Best of luck with it!
