Mr Rennie made the pledge during a campaign visit to Kirkintilloch, where he was joined by the Lib Dem candidate for East Dunbartonshire and Business Minister Jo Swinson.
The Scottish Lib Dem leader said his party would “cut less than the Conservatives and borrow less than the SNP and Labour”, with its plans for the public finances.
Mr Rennie said: “It is only because we have a costed, responsible plan to balance the books by 2018 that we can give an £800m boost to the Scottish NHS.
“It’s because our plan to balance the books is one with a heart as well a brain.
“The Conservatives want to cut too much, taking us back to 1960’s levels of public spending. The SNP want to borrow even more than Labour, recklessly putting at risk the decent funding of public services.
“Our plan to cut less than the Conservatives and borrow less than the SNP and Labour enables us to commit to building a stronger economy and a fairer society.
“Liberal Democrats have been first out the blocks in setting out how we would deal with the deficit. The SNP, Labour and the Conservatives must be open with people in Scotland about what is needed to deal with the deficit fairly by 2017/18.”
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