This client has paid up in advance, trusted you then you renegade on his booking for a longer booking.
This is not fair, and no clients are not there to borrow money from.
Replacing with a larger booking puts your agreement with him at risk, so the two hour is worth loss of his pre-payment?
If I take deposit or payment in advance I don't class it as mine until the booking has happened, otherwise I am in debt to that person - also the booking is set in stone.
Clients are not loaning machines.
Just read the OP again.
Do you mean an installment was due, which you pay back from cash you have earned from other bookings, during your booking with him - so now he wants the installment you would have paid in cash in the booking?
Is that how it works?