It's not your fault, and you shouldn't pay. I'm quite surprised other border started talking to you about it instead of going to BO. Also since BO was fully aware of situation and that horse has sliders on he/she is responsible for the accident in 1st place.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."