The IRS sets an interest rate each quarter that is applicable to underpayments by individuals. The rate for the third and fourth quarters of 2020 is 3%. You can see all interest rates listed by the National Association of Tax Professionals at https://bit.ly/3bVcCmw.
A sale of property that allows for tax deferment if at least one payment is received after the end of the tax year in which the sale occurs. The installment method does not apply to year-end sales of publicly traded securities. Dealers may not use the installment method. Investors with very large installment balances could face a special tax.