The sentence discusses Bob's desire to increase his earnings to afford a house.
While "money" is general, "salary" or "wage" are specific forms of payment for work.
"Salary" usually refers to a fixed amount paid monthly or annually, while "wage" is often hourly or daily. Both relate to earning more from work. Given the context of wanting to buy a house, which implies a stable income, "salary" is a strong candidate. However, "earn more" can apply to both. "Money" is the most general term for what one earns. The phrase "earn more money" is very common.
Considering the options, "money" is the most fitting general term for what one earns to achieve a financial goal like buying a house.