Add a bullet point to the Glossary, right before the one that begins “Superficial differences between the spelling…”, reading:

Unless otherwise specified:  When “X” is a number, to spend X of a numeric value “V” means to subtract X from V (i.e. replace V with V-X); no action may be taken which requires spending X of a numeric value when the subtraction would result in a number which is illegal as a replacement for that value (e.g. if the value represents a variable which is restricted to non-negative integers, but the subtraction comes up negative).

In practice, this may how we already manage things, but I don’t think specifying it in the Glossary creates unnecessary clutter.  Besides, without something like this, I’ll freak out every time there is a rule that doesn’t specify “A player with at least 50 pesos may spend 50…”, for example.