Resignation conflict according to my employer

Good Day. Nagresign ako last August 30, 2017. I followed the 30 days notice. Ang problem ay before I resigned may naka schedule na convention and the fee was shouldered by my employer. The scheduled convention was run from August 23-25, 2017. I have to attend to this because first until August 30, 2017 I am still the supervisor of my department and second my name was the one written on the check to be paid dun sa convention. The problem is my previous employer is asking me to pay the amount to them dahil hindi na dapat ako umattend sa convention. My problem here is if they knew that I am going to resign diba dapat gumawa sila ng memo to cancel the check or at the very least inform me to delegate the attendance of convention sa iba. Additional information, wala po akong pinirmahan na training contract kaya wala silang habol na kung ano man. Mayroon po ba itong laban sa NLRC kapag nagfile ako ng case. Baka po kase ihold din ang aking certificate of employment. Hoping for your response. Thank you.