module Spree
  module DisableNegativePaymentModeAndSetBalanceAbility
    def self.included(klass) 
      klass.class_eval do
        attr_accessor :disable_negative_payment_mode
       
        validates :payment_mode, :inclusion => { :in => klass::PAYMENT_MODE.values }, :unless => :disable_negative_payment_mode
        validates :payment_mode, :inclusion => { :in => klass::PAYMENT_MODE.values.select { |value| value >= 0 } }, :if => :disable_negative_payment_mode
    
        before_validation :set_balance, :on => :create
      end
    end

    def set_balance
      self.balance = user.store_credits_total + effective_amount if user && amount
    end
  end
end
module Spree
  module DisableNegativePaymentModeAndSetBalanceAbility
    def self.included(klass) 
      klass.class_eval do
        attr_accessor :disable_negative_payment_mode
       
        validates :payment_mode, :inclusion => { :in => klass::PAYMENT_MODE.values }, :unless => :disable_negative_payment_mode
        validates :payment_mode, :inclusion => { :in => klass::PAYMENT_MODE.values.select { |value| value >= 0 } }, :if => :disable_negative_payment_mode
    
        before_validation :set_balance, :on => :create
      end
    end

    def set_balance
      self.balance = user.store_credits_total + (effective_amount(amount) || 0.00) if user && amount
    end
  end
end

Update set balance method for effective amount
Update set balance method for effective amount