Rambam Laws of Shmita and Yuval

Hello Rabbi,
Maimonides writes in the Laws of Shemita and Yuval, Chapter 1, in Halacha 1:
It is a positive commandment to rest from working the land and working the trees in the seventh year, as it is said (Leviticus 25:2): "And the land shall keep a sabbath to Me," and it is said (Exodus 23:13): "In plowing and in harvest you shall rest." And whoever does any work of working the land or trees in this year has nullified a positive commandment and has transgressed a non-commandment, as it is said (Leviticus 25:4): "Your field shall not be sown, and your vineyard shall not be pruned."
As you can see, Maimonides chose to quote the verse "in the harvest and in the harvest of
Maimonides believes that the seventh addition is a law passed down to Moses from Sinai, therefore the law of plowing and reaping are taught in terms of the very act of the sabbath itself – plowing is forbidden (apparently by virtue of the act of the Sabbath of the land) and reaping is forbidden.
The question is, how does Maimonides interpret the verse as neither "Ra" nor "R"? Doesn't he have to be subject to them?