below is an excerpt from Sanhedrin 32. If MiDioraisa something is required, how can the chochomim enact something against that???
8. Answer #1 (R. Chanina): Mid'Oraisa, Drishah v'Chakirah applies to monetary and capital cases alike. Chachamim enacted that monetary cases do not require Drishah v'Chakirah due to Ne'ilas Delet (lest potential lenders fear that the witnesses will not pass Drishah v'Chakirah).
Harvey/Mechael Benton, los angeles, usa (golusville)
There is a rule quoted in many places in the Rishonim (i.e. Tosfos in Beitzah 8b, DH "Tani") that Chazal have the right to enact a decree that will bypass a positive Torah commandment (so long as it does not actively transgress a negative Torah commandment). The Posei'ach Sha'ar quotes other answers as well (i.e. Hefker Beis Din Hefker).
All the best,
Yaakov Montrose