obligation on the abstract world (Part I)

Imagine a hot afternoon you've gone shopping in clothing store and bought a shirt
And when he was come home
  You regret buying T-shirts
What makes you still stay on your shopping
Of course, you can win the consent of the seller to reclaim shirt
But you can not terminate the contract unilaterally

And if forced to return to the seller shirt
And to force your money out of the safes seller
Surely punished

Others blame you and they tell you
You should remain on contract
Why?
Why should you be loyal?
You said "I bought this shirt," the clerk said "I sold the shirt."
And the end
The words do not exist now.
If you say must be loyal to the will of contracts that existed when buying
The answer is that
the "will" be the moment of contract
And now does not exist
In addition
If today I buy a house
What reason is there to hold my children contract
While the person is dead
Can not wills
And my will Is not considered my children's will

However  what we hold convention
The correct answer is: to obligation
obligation abstract creature
That is not there in reality
like mountains
nor in the world of mind
Such as justice
It is a creature that person in their dealings assume it