In your story RC incurs no opportunity cost for planting seed in and tending a less efficient field. There should be an interest rate as a function for lending the last Nth percent of the seed based on the opportunity cost of planting and harvesting the less efficient field, which at some point crosses 0 and becomes negative. The interest rate drops even more quickly once his next expected yield will be more than he can eat or store for more than a single planting season.

If RC is currently in the situation where his desired interest rate is still positive for his last unplanted seed then his capital is constrained and he should instead ask for investment seed from S, for which he would be willing to pay interest.

In order not to starve RC should aim to grow sufficient grain such that his probability of having too little is lower than some risk threshold. In most cases this will leave him with excess seeds at every harvest (beyond the extra seeds required to avoid starvation risk) which he can lend. Depending on his assessment of the loan risk, he may even be able to save himself time and trouble by growing less grain to produce fewer seeds with the expectation that his loan will be repaid, which would allow him to realize a profit on an otherwise zero-interest loan.

Likewise, money below a certain threshold should compound; beyond that threshold it represents unacceptable opportunity costs for exercising its power as if it had been used to purchase excess goods. Investing/loaning those purchased goods should be the basis for money's value beyond the threshold.