SAML user mapping to LDAP


I have configured my Nextcloud-Server as SAML Service-Provider.
I already can log in on my Identity-Provider and get redirected to Nextcloud with the logged in User.
I use this Identity-Privder (UCS) as LDAP-Backend too.
My Problem is now: When i login, Nextcloud creates a new User with the login Name entered on IdP. But i want to map this SAML-User to my LDAP-User as both have the same Username and both Services running on the same backend-Server.

Has anyone experienced this issue already and knows how to solve it?

Kind regards Raphi

My Versions:
Nextcloud version: 18.0.7
Operating system: Debian 10.4
IdP: Univention Server
Nextcloud SAML App Version: 3.1.2