Easy, make an Event, with a picture of a closed chest, set the trigger as "Action Key" (That's the Default)
In the trigger, make a switch operation (Not LOCAL!). And create the following Switches: "Chest01Opened"
and "Chest01Empty".
In that page, create a Switch Operation. And pick "Chest01Opened = On".
Now create a new event page, give the amount of cash you want to give the player, and then show the text "You've found 2500 Gold". Then, set for the conditions "Chest01Opened = On". Set the pic of the Event as the opened chest.
In that page, create a switch Operation, and pick "Chest01Empty = On", make another switch operation that says "Chest01Opened = OFF".
Now create a last page, set the pic as a open chest, and choose for the condition "Chest01Empty = ON", And there you. Working chest =)