If you accept the fact that both the NT and OT are of God then there is no contradiction there. This is a matter of presuppositions. If you don't accept the authority of the NT you're going to see contradictions, and if you do accept it then you won't.
What contradictions? I don't see any verse of the NT that explicitly sets aside the Saturday sabbath, and even those who torture the text to find an implicit setting aside can find no authority in the NT for establishing a Sunday sabbath in its place. (Unless you make the mistake of conflating the Sabbath with the Lord's Day.)
There are fairly few places in the OT where YHWH speaks of something as being perpetual. The seventh day sabbath is perpetual. The Aharonic priesthood is perpetual, the covenant to never flood the earth again is perpetual. Perpetuity is not something that God takes lightly in the OT. When he says perpetual, he damn well means perpetual.