Question

When you solve a problem involving money, what can a negative number represent

Answer 1

Means you owe money.  

I have 100 dollars in by bank, I spend 150 dollars.  

100 - 150 = -50 that means I owe 50 more dollars. 

Answer 2

For this case, it depends on the context of the problem.
 Most of the times the negative numbers in these types of problems mean that we owe money to some person or bank.
 For example, if a car costs $ 2000 and I have $ 1500 available, then I must ask for a loan at the bank:
 $15000-2000 = -500$
 It means that I have to pay $ 500 to the bank.
 Answer:
 Amount of money you must pay.