3,433. Using the Same Produce for First and Second Tithe

Hilchos Maaser 7:3

One may designate specific produce as second tithe and then redeem it, after which he may designate the same produce as first tithe for this crop, or as terumas maaser for some other produce.

Hilchos Maaser 7:4

If someone puts produce aside to take tithes from, he can keep separating from this produce on the assumption that it remains intact. He may eat and drink until all the produce that he put aside has become tithes, at which point he gives it to a Levi. If he later finds that the produce he put aside is gone, he must tithe again doubtfully, but not with certainty (because maybe the produce still existed when he designated tithes from it).