The Minnesota Orchestra sweetened its offer to locked-out musicians Thursday, after an 11th-hour fundraising effort led by Marilyn Carlson Nelson, one of Minnesota's wealthiest people.

The latest proposal in the bitter yearlong dispute includes a $20,000 one-time bonus to each musician, to help offset a pay cut that would reduce base salaries over three years, ending at 25 percent below current levels.

Money for the bonuses would come from the Carlson Family Foundation, 14 other Minnesota foundations and the community group SOS: Save Osmo.

"We consider this a unique offering, born of shared respect for the Orchestra and in recognition of so many Minnesotans committed to finding a solution," Nelson said in a statement.

The board asked the musicians for a vote before the offer expires at noon Monday.

Music Director Osmo ­Vänskä has said he needs musicians rehearsing next week to prepare for November concerts at Carnegie Hall. If those concerts are canceled, Vänskä has said he would resign.

Blois Olson, a spokesman for the musicians, complained that the board had gone public with this new proposal. "We are further offended that they have again broken the confidentiality of the mediator's process," Olson said, referring to former Senate Majority Leader George Mitchell's offer to mediate the negotiations. "We encourage management to stop playing games and work through the mediator."

In an interview, board negotiator Doug Kelley said the proposal was made directly to musicians and not through the mediator.

Season was to start Oct. 4

The orchestra was supposed to begin its subscription season the weekend of Oct. 4. A swift contract deal would preserve the entire 2013-14 season and the Carnegie Hall shows, according the to the board. All of last season was canceled.

The bonus money — $1.68 million, based on 84 musicians — is the newest development in a contract dispute that has gone on for nearly 18 months. The bulk was achieved through fundraising efforts by the Carlson Family Foundation. Marilyn Carlson Nelson, a life director of the orchestra, is vice chair of the foundation and retired chair and CEO of Carlson Companies. According to several sources, she called a meeting of foundation leaders about 10 days ago and asked for help. Fifteen foundations contributed.

In addition, the community-based group SOS-Save Osmo has said it would contribute an unspecified amount to the bridge funding.

Musicians were locked out Oct. 1, 2012, after rejecting a proposal that would have immediately cut base salaries by 32 percent. An offer made this summer sought to cut the base by 25 percent. That, too, was rejected. The terms of a third proposal on Sept. 14 were not made public, although musicians said it was not materially different from the second offer.

This proposal cuts minimums by 18.6 percent in the first year and then by additional smaller increments, to reach 25 percent by the third year. Current minimum pay is $113,204, according to management. That figure would be cut to $84,915 in year three.

The $20,000 bonus mitigates dollars lost in the contract by the musicians. The orchestra board says the proposal would mean a net 17.7 percent reduction in average salary.

The proposal includes revenue sharing if the orchestra exceeds budget goals, and stipulates that president and CEO Michael Henson will accept the same salary reductions as the musicians. Based on the orchestra's 2011 tax return, Henson's total compensation was about $390,000.

The board's release said the three-year proposal would result in a deficit of $3.6 million over three years.

"Our aim was to eliminate our $6 million annual deficit entirely," said board Chair Jon Campbell.