George Osborne is no longer chancellor of the exchequer, but his spirit thrives in the budget presented by his successor. In terms of fiscal decisions that have a material impact on British citizens in the coming years, new measures announced on Wednesday make up a tiny proportion of the whole. It is austerity designed by Mr Osborne and nodded through by Philip Hammond that will shape the economic and political landscape for the rest of this parliament and the next.

So Mr Osborne is a consequential figure despite his relegation to the back benches. His status as a former cabinet minister is not constitutionally recognised but still significant. It carries authority that Mr Osborne can either cultivate or squander. In taking a £650,000 per year salary for a part-time job with a fund management company, Mr Osborne has signalled that he is uninterested in his reputation as a former steward of the nation’s finances, except insofar as it can be turned to the betterment of his personal finances. In serving himself he does a disservice to Westminster.

MPs are allowed to hold other jobs as long as the additional income is declared. It is from those declarations that we know how much Mr Osborne is paid by BlackRock for the four days each month that he works for them instead of his constituents. We also know that he has been paid nearly £800,000 for speeches to financiers since leaving the government. Five short speeches made in the Commons during the same period are covered by his £74,900 salary as an MP.

There is nothing illicit about this arrangement, but there is something distasteful about the architect of many people’s hardship cashing in when there is still so much pain yet to come. As the Institute for Fiscal Studies noted yesterday in its evaluation of the budget, cuts to employment support allowance and tax credits that take effect this April will be felt more keenly and by more people than the rise in national insurance contributions (NICS) for some self-employed workers – including, as it happens, Mr Osborne – that has become the object of a Tory rebellion.

The reform itself is rational and proportionate; Mr Hammond’s green light for cruel benefit changes is worthier of opprobrium. But the NICS measure is also politically fissile because it contravenes a 2015 Conservative manifesto pledge. And while manifestos are not widely read and often disbelieved, the promises they contain have a pseudo-constitutional function. The House of Lords does not, by convention, thwart measures that the government can say are electorally mandated by virtue of their publication in a manifesto. More generally, it matters if governments ignore their pre-polling pledges because there is no other documented contract between those who elect and those who are elected.

There are sometimes grounds to say that events supersede old promises. And Brexit is a big enough event to justify recalibration of most policies. There are also grounds sometimes to regret that unkeepable promises were made. The pledge not to raise taxes was a short-sighted gimmick and Mr Hammond was not its author. But he was elected to honour it and he is unwise to imagine that he can escape dishonour by breaking someone else’s foolish promise. He should admit that the plan since 2015 has changed and show humility about it. Only then can he expect new policies to be judged on their merits.

These are points of convention and protocol, not law. But such things matter as part of the soft power of a constitution that has evolved over centuries. There are habits, courtesies and unwritten codes of decency and honour that should govern the behaviour of politicians. Mr Osborne’s crass course of self-enrichment is a breach of that code. So is flagrant breaking of manifesto promises. They are misdemeanours, not crimes, but they have a cost in corroding public trust and demeaning politics.