"as an investment expense."

Let me try to explain this for you. This is some basic info.

When you make an investment, you can see what you got for the money. That is a value, and it required money, but that is expenditure, not expense. When you pay for a building, you no longer have the money, but you have the building.

An expense is, for example, something that is paid for and then all used up, such as Monthly rent, printer paper, utilities. It also is cash flow, but it doesn't buy something that has what the IRS calls "a useful life." You pay rent Monthly.

A building lasts a lifetime, usually.

"money used for the down payment"

The next word to learn is Basis. When you invest in something, you are putting in your cost, or Basis. As you improve a tangible asset such as a building, the repairs will be expense but the expenditures that make a significant change or improve the building are increasing your investment in the property, so that adds to Basis.

"taken from his contractor's business bank account"

You need to know the business entity type, to know what "taken" is categorized as.

"is deductible from his contractor's business"

Is it part of a business endeavor? For instance, a run down building to be rehabilitated and then sold. Or, to be rented and occupied. Or, as asked, perhaps the business doesn't even own the building, so you can't "put it in the business" if it is owned personally.

These are things you want to learn about, if you are going to prepare taxes for clients.