Can you spend any buff from any unit you control to save a unit that's about to die, or can you only save a unit by spending a buff that is on that specific unit?

Ruling: You can only save a unit by spending a buff that is on that specific unit. You cannot spend a buff from a different unit to save another unit.

Nuances:

• The key phrasing is "spend its buff" (must be the unit's own buff) versus "spend a buff" (which would allow spending any buff on a unit you control)