The agreement is kinder when both people know what happens if repayment is delayed.

Everyone imagines the version where it goes to plan, which is why the awkward part gets skipped. If the money is late, nobody knows what the rules are, so the lender improvises a reminder that sounds colder than intended and the borrower hears a threat where none was meant. Agreeing in advance removes that improvisation. It also makes a delay a known event rather than a betrayal.

When you set the terms, add the one that covers a bad month: when to tell each other, whether the date moves, what you will both do. Say it while the mood is warm. A missed payment then costs money and nothing else.