They had the check he paid to the two brothers. He paid them for something.
"Consulting"
The $3500 check was for workout training. (that's why it was in the news). He paid that way before the incident. THEN, he paid them $500 to go buy the rope, masks, etc. and meet him by the Subway just prior to them leaving for the airport. That is the part that the news keeps forgetting to mention.