If he borrows money and promises to repay, or otherwise incurs a debt, then he should do all he can to repay that debt. To help him in this, and to be sure that there is no misunderstanding on either side, it is wise to have a written record, showing the amount, how it is to be repaid, and so forth.

Of course, if problems arise later, it does not seem reasonable that the debtor should complain that the interest rate is too high, if he freely and voluntarily agreed to it originally. Here again, it is wise to have the terms of the loan in writing, to avoid misunderstanding afterward.