NICOLA Sturgeon was full of praise for Alex Salmond when she appeared on ITV's The Agenda on Monday night. He was a "big, big asset to Scotland," she said, adding: "He's got a particularly direct way of saying things." The First Minister might well have been reading her predecessor's article in the independence-supporting National newspaper published earlier that day, in which he discussed the normally arid issue of the fiscal framework that will surround the new powers to be devolved to Scotland under the Smith Agreement.

In particular, Mr Salmond addressed the 'no detriment' principle, intended to ensure the deal is implemented fairly.

He quoted from Chapter 1 of the agreement where it says the new powers "should ... cause neither the UK Government nor the Scottish Government to gain or lose financially simply as a consequence of devolving a specific power".

The former First Minister interpreted this as a guarantee that any changes to Holyrood's budget as a result of Scotland gaining full fiscal autonomy would be ironed out through the framework to be agreed between the two governments.

Skirting delicately around the IFS's forecast that a fiscally autonomous Scotland would be £7.6billion worse off in the current financial year, he said talk of a "supposed" black hole in the finances was "rendered ridiculous" by the no detriment principle.

It would apply, he said, "whether a lot is devolved, as the SNP proposes, or just a little is conceded, as the London parties suggest".

He also said it would operate in a year when Scotland was in a stronger or weaker budgetary position than that of the UK as a whole.

Mr Salmond's interpretation of the no detriment rule was swiftly shot down by the UK Government, which quoted the part of the Smith Agreement saying it applied specifically to the package of powers agreed between the parties last year.

Chief Secretary Danny Alexander it more succinctly. Asked if no detriment would also apply to a full fiscal autonomy set-up he reasoned: "That is not fiscal autonomy then, is it?"

He said the rule was designed for a system in which resources continued to be pooled and shared around the UK, as they are and will continue to be under the Barnett Formula.

In fact, the no detriment principle is designed to achieve two things.

Firstly, it should ensure there are no winners and losers at the moment powers are devolved. When Holyrood takes control of all the income tax raised in Scotland, for example, its block grant from Westminster will be reduced by the amount the Treasury foregoes, around £11billion.

Similarly, when Holyrood takes charge of attendance allowance and other benefits, it will receive the sum of money previously spent on them in Scotland by the DWP, approximately £2.5billion.

The second purpose of the no detriment principle, far from keeping budgets the same, is to establish ground rules by which they can change, but change fairly, so the two governments benefit or suffer from their policy choices independently, without their decisions impacting on the other.

For example - and it is but one among many - some benefits to be devolved to Scotland entitle recipients to a cut in their road tax, which remains a UK levy. The Scottish Government could cost - or save - the UK Government money by making changes to Scottish benefits. A system of compensation, in either direction, will need to put in place.

The fiscal framework will cover just such issues and it fair to say the inter-government negotiations, when they eventually get underway, are likely to be fraught.

But in the meantime, Mr Salmond has raised the stakes considerably in the debate over full fiscal autonomy. His opponents, he claims, will be "explicitly reneging on what they solemnly signed last October" if they refuse to accept his interpretation of the no detriment principle. When SNP MPs press for full fiscal autonomy in the next Westminster parliament, as Nicola Sturgeon has promised, they will fail. Labour, the Conservatives and Lib Dems will protest loudly that it would plunge Scotland billions of pounds deeper into the red. What's the betting the Nationalists will turn round and claim their opponents put impossible obstacles in the way of full fiscal autonomy by going back on their commitment to the no detriment principle set out in the Smith Agreement?