Spain's Economy minister, Nadia Calviño, is sure that her government's proposed 2019 budget will go down well with the EU Commission. Although many measures in the spending plan depend on reduced deficit-reduction targets with Brussels, Calviño foresees no problems: "We are convinced that this plan is in line with what is required," she announced on Monday. Treasury minister María Jesús Montero backed her up, adding that no one in favour of "improving people's lives" would reject the 2019 spending plan.

It will chiefly be Montero's task to lead Spain's 2019 budgetary negotiations with the EU. But Calviño can surely help her to secure a favourable hearing for the Spanish government's proposals in Brussels: before she was appointed Economy minister in June, Calviño led the EU Commission's budget department.

Indeed, for Pedro Sánchez - a prime minister concerned to increase Spain's influence within top Brussels institutions - it was Calviño's EU credentials that qualified her for a position in government. But now, it's precisely those starry credentials that threaten to cause a conflict of interest.

Calviño has yet to officially step down from her EU job. Instead, she has taken "unpaid leave on personal grounds", as an EU spokesperson said after she assumed her role as Spain's Economy minister on 6 June. The spokesperson also confirmed that Calviño's membership of the institution's social security scheme had been suspended and that she won't be eligible for a pay rise.

This point-missing statement neglected to mention the key point - namely, the conflict of interest that arises from Claviño retaining her EU job (even if only nominally) at the same time as running the Spanish economy.

New EU guidelines announced in January extended the "cooling-off" period for commission officials taking up new posts outside the institution, from eighteen months to two years. During this time, former commissioners will be "subject to restrictions in certain activities, such as lobbying members of staff or the commission". Such restrictions, one hopes, will be applied to Calviño as she sells Spain's 2019 budget to her former colleagues in Brussels.

Are they her "former" colleagues, though? According to Calviño's CV on the EU Commission website, she is still the director general of the organisation's budget department ("2014-date", it reads).

Yet the EU's updated code of conduct stipulates that "commissioners avoid situations of conflict of interest, [as well as] situations that might be seen this way".

We are left to wonder, then, why Calviño hasn't resigned from the EU, instead taking "unpaid leave" from the institution when appointed as Spain's Economy minister.

Perhaps Calviño believes that her new governmental role will be short-lived. Or perhaps she's waiting for the "right moment" to renounce her EU post. That moment may well arrive - coincidentally, of course - only when Spain's 2019 budgetary negotiations with Brussels are complete.