Finance Minister Joe Oliver won't say how the sale of GM shares will help the Conservative government post a pre-election surplus, but promises to lay out the details with his April 21 budget.

For the second day in a row, Mr. Oliver was asked whether the stock sale announced this week will make the difference between Ottawa being in the red or in the black.

"We did not need to sell those shares to balance the budget. We'll have all the numbers in when I present the budget," Mr. Oliver said Friday in an interview with CP24.

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Mr. Oliver muddied the waters a day earlier by suggesting the government could have used an accounting move to book the GM sales without actually selling them.

"We would have been able to take into account a prospective sale price even had we not sold the stock when we did. That's an accounting provision that was permissible," Mr. Oliver told reporters on Thursday.

Melissa Lantsman, a spokeswoman for Mr. Oliver, insisted Friday that under either of those two scenarios, the revenue gain would not be the difference of whether Ottawa forecasts a surplus.

The Conservative government has made a return to balanced budgets a central political promise. This week, Mr. Oliver announced that he would be bringing in balanced-budget legislation that would compel future governments to avoid deficits unless they face a war, natural disaster or serious recession.

Mr. Oliver was asked several times Thursday whether the government could still have projected a surplus without booking the sale of GM shares.

"I don't have numbers now, but we will divulge them in the future and you can determine the amount," he said in French. "It's nothing very complicated," he added later.

The federal government sold its shares in General Motors to Goldman Sachs for $35.61 (U.S.) each. The shares were originally obtained as part of Ottawa's contribution to the government bailout of GM during the global financial crisis.

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That sale would translate into a federal revenue gain of about $3.287-billion in Canadian dollars. However, some of that amount has already been recorded.

The government had previously booked the value of the shares at $15.31 (U.S.), meaning the fiscal gain this year would be the difference in share price. It would also depend on how the money is converted into Canadian funds. A rough calculation would put the fiscal gain at about $1.9-billion in Canadian dollars, the same amount Mr. Oliver projected as a surplus for 2015-16 with his Nov. 12 fiscal update.

Another complicating factor is that Ottawa has also already booked $1.5-billion for 2015-16 for asset sales generally and it is not clear how that will impact Mr. Oliver's recording of the GM shares sale.

Peter DeVries, a former senior official at finance, said the accounting of assets is a complicated issue and disputed Mr. Oliver's claim that an asset sale could be recorded without actually taking place. Mr. DeVries noted that while he expects the budget will forecast a surplus for 2015-16, it won't be known if that actually occurred until the fall of 2016 when the Public Accounts are released.

"That will be well after the 2015 election [before] we'll be in a position to know whether they made it or not," he said.

NDP MP Peggy Nash said she finds it "suspicious" that the government delayed the budget until April and then waited until after April 1 – the start of the fiscal year – to sell the GM shares.

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"It's become clear that they're going to do whatever they need to do to have the numbers reflect what they need for partisan purposes," she said. "This is a government that is setting new lows in fiscal transparency."