The enforcing entity here is the central bank they both have accounts with. If a customer of Monzo sends £10 to a customer of RBS, then that money never leaves the central bank, and both records are just updated. But the total amount in the central bank must still add up to the accounts of both Monzo and RBS, otherwise there is a discrepancy.
The settlement process through a central bank is a way of ensuring that banks dont need to literally send truckloads of cash to each other at the end of the day.
Monzo says to the central bank, "today I sent RBS £1,500,000", and RBS says to the central bank, "today I sent Monzo £1,200,000". So the central bank just debits Monzo's account with them by £300,000, and credits RBS's account with them by £300,000. The total amount in the central bank remains the same.
So, sure, a bank could claim they sent less money to another bank than they did, but eventually the numbers wouldn't add up, and it would trigger a bucketload of auditing, likely resulting in revocation of banking licenses, and legal issues for both the bank and people involved.
The settlement process through a central bank is a way of ensuring that banks dont need to literally send truckloads of cash to each other at the end of the day.
Monzo says to the central bank, "today I sent RBS £1,500,000", and RBS says to the central bank, "today I sent Monzo £1,200,000". So the central bank just debits Monzo's account with them by £300,000, and credits RBS's account with them by £300,000. The total amount in the central bank remains the same.
So, sure, a bank could claim they sent less money to another bank than they did, but eventually the numbers wouldn't add up, and it would trigger a bucketload of auditing, likely resulting in revocation of banking licenses, and legal issues for both the bank and people involved.