Specific problem of army of Spain was twofolds:

- logistical difficulty to carry cash from France to Spain, through the pass of Pyrenees and across sierras.
Please note that the cash flow was going to Spain for 5 years, which contradict the "conquest to get money" hypothesis.

- reluctance of Napoleon and french government to fund a full scale war, against multiple weak adversaries, who made any decisive strike impossible. War in Spain was thought as a 17th century territorial control matter, with local offensive to seize major fortress and long idle period in winter.
The two main exceptions wer Massena's offensive in Portugal, and the coordinated effort of the marshals to counter-attack Wellington after Salamanca.

So arrears had more to do with political situation than with a financial pattern.