The Mishna (Maaser Sheni 3:2) rules that one should not buy Terumah with Maaser Sheni money since one thereby further limits who can eat the Maaser Sheni which could lead to it being wasted. The Gemara there extends this to buying Sheviit produce with Maaser Sheni money, and the Rambam rules this way as well (Maaser Sheni 7:13) since one has added the restriction of having to finish the produce before Biur Sheviit.
Now, Maaser Sheni is taken in years 1,2 and 4,5 of the Shemitta cycle. They must be consumed by Pesach of year 4 and 7 respectively (Maaser Sheni 5:6). Accordingly, during Shemitta one could only have Maaser Sheni money from a few years ago that must be consumed by Pesach.
Biur Sheviit takes place once the Shemitta year's crop of that species is no longer available naturally in the fields (Rambam Shemitta 7). Seemingly that usually occurs towards the end of the 7th year or even into the 8th year, but not commonly before Pesach of the 7th year.
Accordingly, what case is there where adding Sheviit restrictions to Maaser Sheni would end up with a greater restriction on consumption? The answer I came up with is using Maaser Sheni money taken from produce from the very beginning of the 8th year to buy leftover Sheviit produce that hasn't quite reached the time for Biur.
But I couldn't find anyone who says that nor anyone who says the prohibition on buying Sheviit produce with Maaser Sheni money doesn't apply to old money used before Pesach of year 7. Does anyone say that or offer a different resolution?