help for next step

in my project me try to deduct total amount of customer via rfid card for example : the total amount in rfid card is 2000 , billing amount is 600 how to deduct these amount and remaining amount steady in my one fid card

plz advice me

In system there are three cards used. Those three cards are used for things and one extra card I want to use for payment purpose. In that card,billed amount should be detected and remaining amount should be displayed on LCD and MOBILE too.

my coding file attch below

coding.ino (7.82 KB)