How to mock offset method using RSPEC?

Please don't downvote. I am new to RPSEC.

I've a method named _apply_offset. It contents are as follows-

def _apply_offset(campaign_params)

if campaign_params['adsets'].length <= 0
  return campaign_params
end


## offset
offset = Facebook::Currency.new(current_advertiser).offset
puts "offset = "+offset.to_s
campaign_params['adsets'].each do |adset|
  # lifetime_budget,daily_budget
  if adset['budget']['value']
    adset['budget']['value'] = (adset['budget']['value'] * offset).round
  end

  next if(adset['bid']['method'] == 'OCPM')
  if adset['bid']['value']
      adset['bid']['value'] = (adset['bid']['value'] * offset).round
  end
end

return campaign_params

end As you can see in the above code I am calling Facebook::Currency.new(current_advertiser).offset

I want the value of this function to return 100.

Offset function looks like this-

def offset
  currency = M_FACEBOOK_CURRENCIES['JPY']
  currency = M_FACEBOOK_CURRENCIES[@facebook_information.currency] unless @facebook_information.currency.nil?
  return currency[:offset]
end

my RSPEC code looks like below-

describe "#_apply_offset" do
let(:current_user) { User.find(10) }
let(:current_advertiser) { Advertiser.find(4) }
let(:campaign_params) {
  {
    'adsets' => []
  }
}

before do
  allow(controller).to receive(:current_user).and_return(current_user)
  allow(controller).to receive(:current_advertiser).and_return(current_advertiser)
end

subject { controller.send(:_apply_offset, campaign_params) }
context "CPC/CPM params" do
  let(:campaign_params) {
    {
      'adsets' => [
        {
          'budget' => {
            'value' => 1000
          },
          'bid' => {
            'value' => 1000
          }
        },
      ]
    }
  }
  describe "budget value" do
    it "offset budget value and round" do
      expect(subject['adsets'][0]['budget']['value']).to eq(1000)
    end
  end
  describe "bid value" do
    it "offset bid value and round" do
      expect(subject['adsets'][0]['bid']['value']).to eq(1000)
    end
  end
end
context "OCPM params" do
  let(:campaign_params) {
    {
      'adsets' => [
        {
          'budget' => {
            'value' => 1000
          },
          'bid' => {
            'method' => 'OCPM',
            'value' => ''
          }
        },
      ]
    }
  }
  describe "method = OCPM" do
    it "expects round method doesn't execute" do
      expect(subject['adsets'][0]['bid']['value']).to eq('')
    end
  end
end

end

In the before hook i tried allow(Facebook::Currency).to receive(:offset).and return)(100) but it doesn't work.

Please help.