The extra cash was only revealed last week when the SNP used it to buy off the Scottish Greens.
The £125m underspend, plus £60m in the business rates pool is far more than the £108m the Scottish Government says it will collect by setting higher thresholds on income tax than the rest of the UK.
That decision is now set to cost jobs and stymie growth, amid warnings from business leaders that it will put off investors and firms from committing to Scotland.
Finance spokesman Murdo Fraser said:
"Throughout this entire budget process, Derek Mackay has tried to hide the facts from taxpayers in Scotland. Now we know why.
"It turns out Mr Mackay had more than enough money to pay for his budget and pass on a tax break to working families. Yet he opted instead to keep the cash in his back pocket in order to buy off the Greens.
"The SNP could easily have turned to us for support in giving taxpayers a break. But Nicola Sturgeon is a First Minister who would rather stitch up a deal with her independence supporters in the Green party than give taxpayers a break.
"We are about to become the highest taxed part of the UK. Now we know: it is not just bad government, and bad policy - but it is utterly unnecessary as well."