mmm Deal or No Deal? springs to mind.... give him a box with 20K written inside it...
I would not accept it.
1: You receive 10K
2: To sell goods, you first got to buy goods.
3: What goods do you sell & where can you buy them
4: They would need a 100% markup to double your money
5: Where do you go to be able to sell them all in 1 month
6: You don't sell all your goods and haven't doubled your money
7: You now owe 10K to your father in law and you're left with stock you dont want nor need
8: You starting to get my point? When you could skip the hassle and forget it.
Its a pointless exercise unless you was going to use the money to start your own business, and if you were - try looking around for franchises where you buy a business with proven methods of making profit, so long as you have the money to invest, time, energy & commitment.