The Post reports that City Council Speaker Christine Quinn wants to spend $20 million giving developers of unsold condos up to $50,000 a unit if they’ll turn “market rate” homes in Harlem and Brooklyn into “affordable housing” homes.

In one example, a developer would take the payment for selling a home supposedly worth $500,000 for $300,000.

If it goes through with this plan, all the city is doing is slowing market forces as they turn overpriced housing into affordable housing without the government’s help.

Developers and their financiers can hang on for a while, sure, keeping condos empty. But at some point, they’ll have to throw in the towel, selling off the units at their real value (hint: likely not half-a-million dollars for a no-frills box in Harlem).

Yes, there’s a problem right now with such sales. Mortgage lenders won’t approve loans for buyers in a building where most units are empty, because the lenders worry (for good reason) that without enough owners to pay upkeep fees, the property against which they’re lending with deteriorate.

But this is not an unsolvable problem. It just takes time, time for developers and their financiers to capitulate.

Vulture buyers can cobble enough money together from investors with their own cash to purchase entire blocks of units in a building at super-low prices. The investors can then flip the units to buyers for a profit, providing their own financing for such buyers — and once the investors reach a magic number of units sold, the regular mortgage lenders will be back.

Quinn’s solution works against the process, because it gives opportunistic investors yet more uncertainty, and thus more reason to delay. Investors will have to worry, for, example, that lower-income buyers will deter other buyers from purchasing units in the rest of the building, or that the lower-income buyers won’t be able to pay the upkeep charges of the building without unpredictable government subsidy that comes with strings attached.

Plus, the condos’ current owners — the developers and the banks — now have yet another reason to delay what should be inevitable, in the hopes of avoiding it. Why not wait, they must think, and try to wring some money out of New York City to pay part of the cost of their huge misjudgments?

Quinn & Co. have a motive, of course. By turning private affordable housing into government-subsidized affordable housing dispensed under social mandates — which often include income limits and preference to people already living in the neighborhood — they’ll get a say in who lives there, and they’ll attract yet more voters who hope to receive direct government benefits.

Ironically, most such voters would be better off letting market forces work. As prices for “luxury” condos plummet, they will pull down prices of older-stock housing right down with them, opening them up to more middle-class households.

So what could Quinn’s $20 million buy, if New York didn’t spend the money on needless payments to developers and banks in a bid to claim political credit for something that’s going to happen anyway?

It could fund all but $5 million of the cost of the MTA’s plan to fix ceilings in subway stations over the next five years (p175).