Troop Upkeep-Reinforcements - Travian v2 Wiki (S2-S6)

Who pays for Troop Upkeep?

If your troops are at your own rally point, they will use your own resources for thier upkeep.

If your troops are at another player's rally point, their upkeep is paid by the owner of the rallypoint they're at, NOT the owner of the troops.

If your troops are returning from another player's rally point, their upkeep is paid by the owner of the troops.

If you attack a village with your troops, you still pay the upkeep cost like your troops are in your rally point.