While cheating Switzerland at the same time, Austria was employing the same tactics to deceive Spain, only with better conditions and more lenient demands.
Though the Duckboard Empire had declined, it was still one of the great powers. It couldn't be bribed with small benefits, and Austria was unwilling to pay a hefty price.
Thus, Austria had to lower its demands; they didn't even need Spain to send troops to war. All Spain had to do was to station its troops at the border to annoy the French, and then, when the overall situation was settled, they could just move in for the killing blow.
If the French won, they would act as if nothing had happened and just carry on as usual.
After all, even the military expenses were covered by Austria, the interest-free loans up front were merely the Spaniards' appearance fees.
It was a transaction with no risk of loss, no need to worry about losing capital.