贡献商城：贡献度兑换的扣减规则

local function exchange(actor, price)
    takeitem(actor, "金元", price, 0, "兑换")   -- 扣错货币
    giveitem(actor, "行会令牌", 1, 0, "贡献兑换")
end

local function exchange(actor, price)
    local c = tonumber(getplayvar(actor, "CONTRIB")) or 0
    if c < price then
        sendmsg(actor, 1, "贡献度不足，还差 " .. price - c)
        return
    end
    setplayvar(actor, "CONTRIB", c - price)   -- 先验后扣
    giveitem(actor, "行会令牌", 1, 0, "贡献兑换")
    sendmsg(actor, 1, "兑换成功，剩余贡献度 " .. c - price)
end