Final answer:
The type of agreement described is an Option agreement in which Roger has the right, but not the obligation, to buy the farm within 6 months.
Step-by-step explanation:
The type of agreement described in the question is a Option agreement. An option agreement is a contract that allows one party (in this case, Roger) the right, but not the obligation, to buy a property (the farm) at a specified price within a certain time frame (6 months). In this agreement, Roger offers $350,000 for the farm and puts down $10,000 as an upfront payment. John agrees to keep the offer open to Roger for 6 months, giving him the option to purchase the farm at the agreed price.
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