N&H ex. 2.7: “I have bought a horse that I may not be tired” is translated as
Equum emi ne fessus sim.
This seems acceptable when the purchase of the horse is supposed to remedy the speaker’s constant state of tiredness, assumed to derive from his copious pedestrian wandering (“…so that I may no longer be tired”). But if the purchase occurred e.g. in view of a future trip, its purpose would be not to become tired as a consequence of the specific journey. Would then sim, which reflects a durable condition, still apply, or should fīam be preferred?