(Devarim 16:10) "And you shall make a festival of weeks (shavuoth) to the L-rd your G-d": Because it is written (Shemoth 23:16) "And the festival of the harvest (Shavuoth) the (time of bringing the) first-fruits of your labor," I might think, if you "harvest, you make a festival, and if not, you do not make a festival; it is, therefore, written "and you shall make a festival of weeks to the L-rd your G-d" — Whether or not you have a harvest, you make a festival.

"the tribute of the offering of your hand": We are hereby taught that one brings his obligatory offering from chullin (non-consecrated monies).

And whence is it derived that if he wishes, he may include some of his tithe monies? From "as the L-rd your G-d will bless you."