I will ask the question using the following scenario: Reuvan wrote a sefer that he was charging for it 50 Shekels. Came along Shimon who wanted to buy the sefer. At that time he didn't have money on ...
Can one give a present on shabbas? If the answer is no, then how does it work when one eats by someones house the food one gets is a present (it is changing reshus). Sources please.