In final normal invoices, when a retainer invoice has been paid, it is shown at the right bottom of the invoice the amount of the retainer invoice paid in red, ok.

In order to be more clear for us and for the customer to remember which retainer it is, could you please add more information about that retainer invoice paid like :

Retainer invoice Number
Retainer invoice date
Retainer invoice % (percentage of the total final invoice)

example :

Retainer already paid : RET-0001 - 01/24/2020 - 50% 10 500 $

Thanks for taking in consideration that request