I'm backing Martial on Betfair

Betfair are offering a special promotion for the Man Utd match. They are paying double odds on the first scorer market.

Check their website here. 

For example if you back Wayne Rooney to score first at 5/1 for €10. Your odds will be doubled. You will be paid winnings of €60 in cash and €50 in free bets.

This offers good value.  What I recommend is 

1. Back one of Martial or Rooney as first scorer. They are both 9/2 joint favourites at the time of writing.
2. If you win wait for your free bet to be credited, then bet on a similar 9/2 outcome with your free bet.

I would probably go with Martial as he is showing better recent form. The bookies know this too though and if they still rate them equally likely to score then they probably are equally likely to score, so pick either joint favourite.

Never use your free bet on short odds. A €50 free bet at odds of 1/10 will pay back €5 almost certainly. But a €50 free bet at odds of 4/1 will pay back €200 about 20% of the time - this is 8 times more lucrative in the long run.

Scenario 1:  You lose the first bet so no free bet. Profit: -€10 Probability: 85% This is the most likely outcome of them all. You will be down a tenner. 9/2 means 9 unsuccessful outcome for each 2 successful. So 9 from 11 chance of losing this bet. 9 divided by 11 gives 82% chance of losing but add a few per cent points to account for the bookie edge. This gives a 85% chance of losing on a 9/2 bet and on the flip side a 15% chance of winning.	Scenario 2:  You win your first bet but lose your free bet. Profit: €45 Probability: 13% (=15% * 85%) You should be winning on a bet like this every now and again. The payout is quite attractive but not high enough to make this a long term winning strategy without the free bet incentive.	Scenario 3:  You win your first bet AND you win your free bet. Profit: €247.50 Probability: 2% (=15% * 15%) This is the incentive that makes it worth while. You win €45 on the real money bet and earn a free bet to the value €45. At 9/2 if your free bet wins then you will have made another €202.50 profit to add to your €45. This is unlikely - but the boost you get 1 in 50 times will more than make up for your lost tenners.
E[Profit] = -€10 * .85 + €45 * .13 + €247 * .02 = €2.98

So there you have it, you have eliminated the bookie's edge and are ready to beat the bookie.  €2.98 looks like a small profit but it is realistic and it can build up over time.

You are only getting a big payout from the free bet about 2% of the time. Without this the long term expectation is to lose €2.70 for each tenner. You will need a bank roll capable of sustaining these losses up to 50 times. That would amount to about €120. In other words expect to lose €120 before finally winning big to the tune of €247.50.

So its not a get rich quick scheme, but by carefully picking similar promotions in the long term you can accumulate a little extra beer money.

Paddy power has another good promotion read here. Also Sky Bet has a very similar one to Paddypower on the live Championship, League 1 and League 2 games. Skybet also give a free €5 bet per week for staking at least €25. So there are some good offers out there. Betfair do a 2/1+ and a 3/1+ on the live dog and horse races. These types of offers are all good. And by playing them all you will probably be making a small profit every month (I find I can withdraw €100 per month from my online bookies).

Stay away from offers like money back as a free bet if your horse is second to a 20/1 shot. In the first place its too unlikely that the 20/1 will win and if he does then its still not a given that your horse will be second - you wont get a free bet back often enough to claw back the bookie's edge that you are losing on your real money punts.

You really want to be getting your money back as a free bet at least 15% of the time.