When I add = into a script, it works, if its + then it doesn't work, why?

Ok, please help; heres the script:

local people = player.opeople.oPeopleOnBoard.Value
	local cash = player.leaderstats.Cash.Value
	
	print(player.opeople.oPeopleOnBoard.Value)
	-- for the thing make money first, so:
	-- player.leaderstats.Cash.Value = player.opeople.oPeopleOnBoard.Value
	 player.leaderstats.Cash.Value = player.opeople.oPeopleOnBoard.Value


	player.opeople.oPeopleOnBoard.Value = script.Parent.Parent.One.Value
	player.tpeople.tPeopleOnBoard.Value = script.Parent.Parent.Two.Value
	player.rpeople.rPeopleOnBoard.Value = script.Parent.Parent.Three.Value
	
	player.PlayerGui.GUI.Main.One_Value.Text = script.Parent.Parent.One.Value
	player.PlayerGui.GUI.Main.Two_Value.Text = script.Parent.Parent.Two.Value
	player.PlayerGui.GUI.Main.Three_Value.Text = script.Parent.Parent.Three.Value
	
	
	local name = script.Parent.Parent.Name

on line

 player.leaderstats.Cash.Value = player.opeople.oPeopleOnBoard.Value

if I change the "=" to a "+" the whole scripts disables. First off, what i'm trying to do is when the player does something, I want the people on board of that vehicle to disembark and pay the driver, thus giving the driver (player) more money when the passengers (npcs) get off, if you don't get that, add the people on board to the money of the player. Help!