In a chapter 13 the buyer would have to not only pay the deficiency owed to you over the 36 to 60 months of the plan, but also would have to make all payments as they become due. I do not know because I do not know all of the facts of the matter, but my gut speculation is that the buyer does not have the money and is stalling for time. Get an attorney who understands land contracts and bankruptcy to pursue your rights aggressively.
This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.