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DENVER — The final day of negotiations between Denver Public Schools and its teachers union will take place Friday ahead of a vote for a potential strike.

The current contract expires at midnight and teachers could vote to strike as early as Saturday.

On Thursday, the Denver Classroom Teachers Association made a counter proposal that reduces $2 million it was seeking for in teacher pay.

The district can accept the offer or submit its own counter offer on Friday.

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Thursday’s meeting was contentious at times. The union alleges someone from the school board was attempting to bargain about the deal behind closed doors.

The district previous proposed offering teachers $23 million in pay, raising their base salary by 10 percent. The union is asking for $28 million.

“The biggest issue now is probably the amount of money,” said Rob Gould, the union’s lead negotiator. “They’ve been grossly underfunding the teachers for the last 10 years.”

The union is still pushing for the district to adopts its salary structure. It doesn’t want teachers to have to wait 10 years for a pay increase to kick in as they pursue a master’s degree or Ph.D.

DPS asked for more time to review the counter proposal.

If no deal is reached, teachers will vote Saturday on whether or not to strike.