The Mishna (Terumot 4:6) writes:
המונה משובח והמודד משובח ממנו והשוקל משובח משלשתן.
One who counts is [more] praiseworthy [than one who estimates]; one who measures is more praiseworthy than one who counts; one who weighs is more praiseworthy than all three of them.
It seems the quantity of the portion is dependent of volume/mass/size.
The Mishna (...
Since the cash back is a proportion of the cash spent, it fits the definition of a rebate (from Wikipedia: a form of buying discount [...] paid by way of reduction, return, or refund that is paid retrospectively.) And one does not pay maaser on rebates.
Indeed Ask the Rav writes (here)
One need not separate ma’aser from rebates, even if they are cash back
Numbers 28--29 is communal offerings. Those were done by Temple priests, and funded by everyone's annual half-shekel dues. (In fact, the Talmud's tractate Shekalim is all about said public funds. These communal sacrifices must come from public monies!)
The average family was responsible for a Passover lamb (which they could "go in on" with other ...
Rivam (quoted in Tosafot Bava Metzi'a 88b) was bothered by your questions, particularly how demai works if this is only a derabbanan issue.
He therefore ruled in completely the opposite direction to Rabbeinu Tam (and Rambam). He holds that produce purchased after mirua'ch is obligated mideoraita, whereas produce purchased before miru'ach is obligated only ...
You have four options
Give it to a Kohen who will burn it for warmth or feed it to his pets
Put it into a dedicated compost pot (not mixed with other compost) and discard it once it has decomposed
Wrap it and discard it in the garbage
Sources: R Moshe Bloom of the Institute for Torah and the Land of Israel (here), SA YD 331:19, R Yosef Tzvi Rimon's ...