I had a thought also about something that Rav Shach [on laws of Truma ] I know does talk about in the Avi Ezri. [I do not recall what he said].
The mishna says one who takes only a fraction of the truma or tithe takes out truma from it but not to a different place. R. Meir said also to a different place. R. Shimshon says the idea is it is truma or tithe but he needs to finish. Example: 100 lbs tevel. He takes 5 lbs tithe. It is tithe but he takes another 5 lbs. The idea is to R Shimshon that the original stack is mixed with tevel and hulin but when he takes more tithe we says he is taking from the tevel.
 Rashi says something similar on a different topic in Gitin 47b.   Jew and gentile own crops together. Tevel and hulin are mixed to R. Yehuda Hanasi. Rashi says you take a tithe and assume you are taking tevel.

The answer for R Shimshon at this point is unclear. I tried last night to think about it but came up with nothing. I ought to mention the person that asks on R Shimshon is the Radvaz. Sometimes it takes a long time for me to come up with an acceptable answer for the baali Hatosfot.