Add or Subtract

I've been considering doing a little project for my community site which allow people to sell and buy players ... They would have a limit of 2,000,000 upon sign up .. However i'm not sure how i'd update this figure .... Say player sells for example Alan Shearer for £300,000 he'd then have 2,300,000 how would i make it add to whats in the db already? same goes for if he was sold and it went down to 1,700,000. Ed