If you want to easily schedule the paying back payment to another Monzo user than this feature being implemented would help (if so, vote for it at the top of the thread):
If scheduled Monzo-to-Monzo payments were implemented then you could click on the transaction for the money your received then click ‘Send money to X’ to set up a scheduled payment to go out on the date you want.
Once the scheduled payment is set up you’d be notified about it just like any other scheduled payment (e.g., a standing order).