A script with a timer would make it less laggy and overall easier, however it can be done with events.
So first thing first we make the system itself.
Let's open a common event, call it say "check balance" we'll put a variable there, set it to 0 as we start the game (flag it so it will only change once, or set to 0 with another event you run at the start of the game, your call)
Set another the same way call it MaxBalance, let's assume it's 1000.
Now this event does this:
When called, it asks to user to input an amount to withdraw (0-max value), then it checks
if (checkBalance + input <= MaxBalance)
-add gold (input)
else
-show message "you can't withdraw that much you can only withdraw *MaxBalance - currentBalance*"
And now we do another event of the same nature, this will be a payback and it's check will be
if (checkBalance - input >= 0)
-Reduce gold
else
-message "You only owe checkBalance"
Now simply connect both those message to a menu set on any npc you wish around the game and the basic feature is done.
Now let's say that rather than a timer (Which could be inaccurate I mean, the player might wander off of his computer) let's count steps.
So you run another common event, this will be triggered by the previous one (turned on when you withdraw, turned off when you deposit enough to cover your pay, and reset the walk count to 0)
Now you want a StepCount variable
it simply goes "if player moved right +1, if player moved left +1...."
and then you put a branch
if (StepCount >= 5000)
-Message: "Those who do not pay their debt stay without a penny"
-Reduce gold (Amount the player owes)
-delete main character weapon and armor
-Whatever other torment you might like
-Set debt to 0
-Set StepCount to 0
-Set this event's running flag to false.
And that should just about do it.
*Side note: It's been a while since I last used RMRK, this should all be possible but I may have overlooked something, but really this should be enough to at least give you an idea*