Agreements between Private Parties
One of the primary concerns in agreements between private parties can be running afoul of usury laws. Usury laws set limitations on the amount of interest which can be charged, and these rates vary by state. States have these rules so that certain institutions or persons cannot take extreme advantage of another person in a less equal financial position.
If you are inclined to loan money, it is best to set forth, in specific terms, the repayment provisions and conditions, in writing. Even when you set forth the terms in writing, you should be aware that this agreement can be later modified by oral agreement, or conduct of the parties just like any other contract. Allowing late payments, non-payment, or partial payment can be construed to be a waiver or modification of the original contract terms, and may seriously affect your ability to collect or enforce the contract, if necessary.