How did the vendor transfer the salt from his sack to the restaurant owner's sack?

Question

A poor vendor, who had just one sack with him, went to a restaurant to sell salt and sugar. However, since he was having just one sack and obviously he could not mix the salt and sugar, so he had poured in the salt first, tied the sack in the middle, and then filled the top portion of the sack with the sugar. But the restaurant owner decided to buy salt only as he did not have any requirement for sugar.
How did the vendor transfer the salt from his sack to the restaurant owner's sack?

Now, there were certain conditions:
If the vender poured the sugar anywhere else than either of the two a sacks, it would become unfit for selling.
The restaurant owner refused to exchange his better quality sack with the old one of the vendor.
No matter how old his sack was, the vendor could not afford to cut a hole in his sack.

Answer 1

At first, the sugar was poured into the restaurant owner's sack. Then it was tied from the middle and turned inside out. After this, the salt was poured into the restaurant owner's sack. Then in this position, it was untied from the middle and the sugar was poured back to the vendor's sack.