Money buys a lot of things, but not always what is needed.

In the case of development policy, cash being funneled into the city’s affordable-housing fund may be purchasing a lot less housing than leaders are hoping it would.

Or, at least, it’s buying them at a much slower pace than required to overcome Oahu’s critical shortage of affordable housing — rentals in particular.

One of the arguments for the elevated rail project as the financing plan underwent review was that the rail alignment would provide the spine for development density, which could yield needed residential units without furthering urban sprawl.

In addition, the value of property within the “transit oriented development” (TOD) zones could be leveraged to enable developers to build affordable rentals along with market-priced units — or the luxury dwellings evidenced in much of the new urban-core development.

Ordinarily, developers find it challenging to make a project comprising only affordable apartments to pencil out for financing. But city and state provisions for land and infrastructure can offset costs; sometimes height limits and rules covering setback and parking are relaxed.

There are “inclusionary zoning” requirements for affordable housing, but many developers find the option of a payment in lieu of building the actual units an appealing option.

Now, however, it’s time to close off some of the escape hatches, where affordable- housing development is concerned.

One recent example of a project where the city has allowed a monetary payout in lieu of the affordable units themselves is a 36-story hotel-condominium tower at Kapiolani Boulevard and Atkinson Drive.

The Mana‘olana Place Hotel and Residential Condominium Project proposes 109 apartment units, 125 hotel rooms, retail and restaurant space.

It also would include a street-level public plaza at the street level fronting the prime location, across from the Hawaii Convention Center.

But disappointingly, the developer, Mana‘olana Partners II, is paying a $2.4 million fee in lieu of delivering the affordable-housing units required of the project, part of its total offering of $7 million in community benefits.

The company is an affiliate of the Los Angeles-based development firm Salem Partners LLC.

It’s clear enough that a $2.4 million payment will not produce housing, with the added value of being built near a transportation hub, equivalent to units being built on site.

On some other site, the cost of land — the most expensive commodity of all — factors in.

This proposal, with the in-lieu payment, recently gained a significant approval from the City Council Planning and Zoning Committee.

Still, it’s not too late for the Council to give more thought to how to shape the project to best serve the community.

City officials also should resolve to drive a hard bargain for the best affordable-housing deal they can get on another Salem project at an earlier planning stage, to be located next to the Walgreens store fronting Kapiolani Boulevard.

At about 35 stories of condominium and hotel units, this project, like the one at Atkinson Drive, would be near the rail line terminus at Ala Moana Center and would be governed by TOD rules.

Jim Ratkovich, Salem’s managing director, said the company is trying to fit its required affordable-housing allotment of units into the project, but also could arrange for those units to be built on another site.

On-site is the best result, bottom line, and sooner rather than later.

Oahu residents who need these units also deserve a chance to live in the convenient locations that rail stops provide. Officials should remember that TOD at rail stops is meant to help the island’s working families easily commute without a car; many of them do so now, by bus.

The Hawaii Housing Finance and Development Corp., the state agency that helps finance affordable housing projects, underscored the basic issue in a 2014 update report: ”Simply put, affordable rental housing is unprofitable, so the market won’t address the need by itself, ” the agency concluded.

“The obvious solution is an effective and productive public-private partnership wherein everyone gets some of what they want.”

What the public wants and needs from these partnerships is that the developers who are capitalizing on rail actually produce some of the units Oahu so desperately needs. Now is the chance to build up that housing stock.

As the much-touted development boom continues to take form in urban Honolulu — including what’s anticipated as part of TOD — the city has to make the most of the window of opportunity to narrow the gap in affordable housing, before that window slams shut again.